right now, over tonightly news. brian williams is up next. on our broadcast tonight, way off course. the mystery has deepened in the search for that missing aircraft. why did it pull a hard left turn before disappearing? and was the electronic transponder turned off? flight risk. those stolen passports some passengers were apparently holding have exposed a huge hole in airport security around the world. accusing the cia. a powerful senator comes out swinging because she says the agency spied on a congressional investigation. caught in the middle. our special report on the worst humanitarian crisis in the world. dr. nancy snyderman on the front lines of the fight to rescue the children of syria. "nightly news" begins now.

good evening. the investigation into that still missing wide body jetliner has taken a sudden turn, because they now theorize the aircraft made a sudden turn after it was last heard from in midair. it's been four days. there is still tonight no trace of a boeing 777 jumbo jet with 239 souls on board. the going theory now is that the aircraft took a hard left, almost reversing course while in flight. the airliner appears to have flown on for at least another hour after its last known location. there were no distress signals. the search has been expanded to cover an area of over 10,000 square miles. we begin tonight's coverage with nbc's tom costello in our d.c. newsroom. tom, good evening. >> reporter: hi, brian. yeah, the facts seem to change with each sunrise in malaysia.

today multiple reports from malaysian officials that the military tracked this plane further than it had previously admitted, and the transponders, they have suddenly stopped transmitting out over the gulf of thailand. as the search for flight 370 continues on the sea and in the air, new questions tonight over what could have happened on board the plane. the malaysian military now says its radars tracked the plane as it doubled back over the gulf of thailand, over the malaysian mainland and a small island in the strait of malacca. the plane's transponder had reportedly stopped broadcasting. cia director john brennan responding to nbc's andrea mitchell today. >> you cannot discount any theory, you know. who had the ability to turn off the transponder? i don't know. how can one's actions be masked, in terms of technologically on the aircraft? >> reporter: the plane's transponders could have been shut off intentionally or as the result of a catastrophic event taking out the electrical system. but veteran 777 captain tom

casey says that would be very unlikely. >> there are ten generators on the 777. it has tremendous super redundancy. you would have to lose all ten generators before the electrical system would take out the transponders. >> reporter: also unexplained, the plane's now confirmed u-turn. experts say only a pilot could do that. so did one of the pilots or passenger hijack the plane? while not ruling out a mechanical explanation, a top malaysian police commander said he's looking at four scenarios. >> one is hijacking, two sabotage, three psychological problem of the crew, and four, personal problem among the crew. >> reporter: and we're learning more about the cockpit crew. >> everyone, this is a youtube video that i made as a community service. >> reporter: an online video

shows the captain in a series of home improvement tutorials, fixing the air conditioning and ice maker, even window sealants. his first officer, 27-year-old fariq hamid, with 2,800 hours of experience. meanwhile, authorities released photos of two iranian men who boarded the plane using stolen european passports. they believe one of them was trying to get to germany to see his mother. malaysian authorities continue to insist neither man, one of them 18 the other 19, is thought to be connected to terrorism. meanwhile, if this plane did fly out over the strait of malacca, it could have headed out over the indian ocean. brian? >> tom costello starting us off again tonight from our d.c. newsroom. tom, thanks. we have a firsthand view tonight from malaysia of this search underway from what so far remains a kind of wide bodied ghost ship. nbc news flew along today on board one of the search aircraft deployed, and now fanning out over a wider area. a huge effort over a vast amount of water. our report tonight from nbc's

keir simmons in kuala lumpur. >> reporter: day after day, this military transport plane has been searching. today we took off with them. life jackets are needed because we will fly so low, sometimes at just 500 feet, scouring the ocean above naval ships, all looking for a plane that simply vanished. we're flying over the malacca strait, the water stretches out on either side of us, you can see ocean on this side, ocean on that side. all day we fly, looking for any clue. we seem to be banking around as if they saw something? the plane turns hard to get a closer look at some debris. false alarm. it took us eight hours to cover 30 square miles. just a tiny fraction of the 10,000 square mile search area. time to head back for now. we're going to keep looking? >> yes, we'll just keep looking.

>> reporter: more flights like this? >> yes, sir. >> reporter: they still call this a search and rescue mission, but so far, it's one with nothing found and no one rescued. keir simmons, nbc news, kuala lumpur. now, back in this country, in washington today, tensions between the cia and a senior member of the u.s. senate boiled over, with senate intelligence chair dianne feinstein, democrat of california, now openly accusing the spy agency of spying on congress. nbc's andrea mitchell, following the story, spoke exclusively today with the director of the cia. andrea, good evening. >> reporter: good evening, brian. this is a knife fight. senate democrats investigating past abuses by the cia, today accusing the cia director, one of the president's closest advisers, of spying on the senate. cia director john brennan under fire today, accused of spying on senate investigators, talking exclusively to nbc news. >> we weren't trying to block anything.

and the matter is being dealt with in the appropriate way. >> john brennan's words -- >> reporter: but in a scathing speech, senate intelligence chair dianne feinstein accused brennan's cia of secretly tracking the senate's investigation into one of the worst abuses in recent cia history, the detention and interrogation under george w. bush, widely viewed as torture. >> i have grave concerns that the cia search may well have violated the separation of powers principles embodied in the united states constitution. >> reporter: feinstein's charge, that the cia hacked secure computers the senate was using, trying to hide the smoking gun of brennan's own policies. brennan oversaw the cia secret prisons and the controversial interrogations of terror suspects under george w. bush. brennan today, at the council on foreign relations, denied he has interfered with the senate investigation. if it is proved that the cia did do this, would you feel that you had to step down? >> if i did something wrong, i

will go to the president and i will explain to him exactly what i did and what the findings were. and he is the one who can ask me to stay or to go. >> reporter: brennan's counter charge in the letter to the senate obtained by nbc, that the senate's staff improperly took sensitive documents from the cia. brennan wrote, it is imperative to learn whether or not a breach or vulnerability exists. the senate says it was only trying to safeguard proof of cia abuses. fearing the cia would destroy the evidence. brennan has an extremely close relationship with president obama. and today the white house stood by him. >> is the president confidant that director brennan has been straight forward with him? >> the president has great confidence in director brennan, yes. >> reporter: the senate torture investigation has been going on for more than five years. now the justice department is investigating whether the cia broke the law by spying on the senate. and also, whether the senate broke the law by secretly taking cia documents. brian? >> andrea mitchell in our d.c. newsroom tonight. andrea, thanks.

as we shift our focus again overseas as we continue our special nbc news coverage of syria's children of war. all across this network, we're trying to put faces to a crisis that this week enters now it's fourth year. the toll of the war has been staggering, a unicef report out today says at least 10,000 children have been killed in this conflict. more than five million children are in need of humanitarian assistance right now. refugees have been fleeing to neighboring countries seeking food, shelter, medical care. throughout the day, our chief medical editor dr. nancy snyderman has been at a hospital in lebanon, near the syrian border, where there are new fears tonight about an old disease that may be making a comeback inside syria. nancy's with us from there by satellite tonight. nancy, good evening. >> reporter: good evening, brian. i'm in a neonatal intensive care unit where, unfortunately, too many of syria's most vulnerable have landed. with a decimation of syria's

health care system, vaccination rates have plummeted from 99% to 54%. that has made even basic health care a casualty of war. the children here, syria's children, now face a new enemy, the threat of infectious disease, including the return of polio. most of the time you're catching the children as they cross the border -- >> yes. >> reporter: -- and starting their vaccinations? >> yes. >> reporter: the urgent effort to vaccinate goes tent to tent. so this means children under 5? but everyone's vaccinated? >> yes. >> reporter: child to child. but today, new fears that the disease may have arrived here there's a suspected case of polio in the hospital. i'd like to show you what that looks like. mom's name is fatma. her husband and sons disappeared about a year ago in homs. she's been alone home with baby. and this little girl, nazine

about ten days ago got a sore throat, stuffed up nose and lost her appetite. and she's very floppy on the right side of her body. this kind of floppiness is compatible with polio, but no one's calling it yet, until the tests are done. thank you, fatma. the world health organization and unicef have taken stool samples, spinal fluid, blood, urine, and they sent it all off to cairo. when those tests are back, we'll know for sure. it has been 24 hours now, and still no word on the results of her tests. and so the wait here continues. downstairs in the emergency room, an 18-year-old arrives seven months pregnant in active labor with twins. but suddenly her painful contractions grow stronger. doctors determine there is no choice but to deliver these babies immediately. minutes later a three pound baby girl. she isn't breathing. and there's a complication with her twin, a sister. complications like this are no longer unusual here.

they have become commonplace among syria's refugees. brian, it's very sad news that the second twin died several hours after being born. basically from overwhelming congenital malformations. there's still a yellow sticker on this isolette, twin number one. this is the first baby who came out. and the real concern is that at just three pounds, how long will she stay in this icu? and then when she's discharged to a refugee camp, how desolate does her future look? her future, in fact, hangs in the balance. brian? >> think of the suffering going on 24/7 that we haven't been able to see, those with no access to care at all. dr. nancy snyderman in the refugee camp just across the syrian border tonight. we wanted to remind you, our special coverage of this humanitarian crisis will continue tomorrow morning on "today," all the time on our website nbcnews.com and on this broadcast tomorrow evening. still ahead for us tonight,

the security flaws that have been exposed during the search for this missing jetliner, and questions about the ids required of passengers around the globe. and later, potentially big news for those who suffer from migraine headaches in this country. a development to report from the fda.

let's go back to our lead story for a moment. as we said before the break, this mystery about the aircraft that is missing, raising new concerns about the identification required to travel by air anywhere in the world. though it appears the two men with those stolen passports on board had nothing to do with this aircraft's disappearance, the fact that they could board the aircraft has now called new attention to a huge lapse in global airline security, even this many years after 9/11. we get our report tonight from our justice correspondent pete williams.

>> reporter: on overseas flights throughout much of the world, a passenger cannot be confident that the person in the next seat used a legitimate passport to get on board. surprisingly, airlines in most countries do not routinely check travel papers against a database of 40 million lost or stolen documents maintained by interpol, the international police agency. >> it's common sense, prudent, good due diligence to make sure that you don't let anyone you don't know -- or carrying a passport that's lost or stolen cross your border and your country or board a plane. >> reporter: interpol commends the u.s. for checking the list more than 238 million times last year, the u.k. 140 million, united arab emirates more than 104 million. the u.s. requires every airline flying here from overseas to get passport numbers for a visa when the reservation is booked or when a passenger gets to the airport. but interpol says fewer than 20 of its 190 member countries bother to do that kind of database check.

as a result, air travelers boarded 1 billion times last year, with no check against interpol's list of stolen documents. an example officials say, samantha lewthwaite known as the white widow of a london suicide bomber and wanted for possessing explosives traveled abroad with fraudulent documents. experts say it's a matter of cost, less concern about terror, and sometimes even reluctance to interfere with a market to interfere with phony documents. >> frankly, there are some countries that don't want this because it could disturb an element of their underground economy. >> reporter: checking the database stops travelers with phony documents from entering the u.s. several hundred times a year, though they won't give specifics. interpol says so many countries are lax, that as many as 40% of passengers on international flights could be using documents that are lost or stolen. pete williams, nbc news, washington. out west tonight, we're just seeing the start of some serious problems following the winter weather we've had, especially in the northern portion of the

nation. there's serious flooding going on in both wyoming and montana, a lot of it because of ice jams. central montana has seen the worst of it, with the red cross now warning of more. in wyoming, the problem is along the big horn river and public officials are watching this carefully, including by drone from above. we are back in a moment with the american president caught between two ferns and a hard place.

residents down on the street level can see this. you can feel it. there is a lot of water, different colors of smoke coming. the scaffolding is starting to tear down, you can hear things crumble within the building, tearing down as the fire goes through it. off explosions here and there. definitely a raging fire and could be awhile before they have it under control with the wind blowing on it. >> felipe there. it's 6:00 if you're just now joining us. a six-alarm fire south of at&t park. kitty corner from the main parking lot of at&t park. dieing aly across the street.

witnesses telling us they feel the heat, feel the smoke. you can see it from different parts of the bay area. this is a building under construction. the whole area is redeveloped, especially in the last few years with so many construction projects. one thing felipe is saying, is that there is the danger now, we seen this before, we've seen this before. the danger of construction projects of embers and sparks going across the street or jumping the block to other buildings. is that what you said? are you seeing and hearing that? >> reporter: that's right. i don't know if you can see what just happened, a large clunk of this building just came in on itself. the flames got higher. there was noise here. i don't know you can see what it did to the flames. this thing is definitely eroding from the inside out. like you said, the