on this sunday night, pure evil has another american is executed by isis terrorists, the president condemns the killing. and the top military officials say the battle against isis will take years. without warning, gas leaks from a texas chemical plant killing four workers and leaving a cloud over a major metropolitan area. kids at risk. the new warning about the popular energy drinks and what can happen if young children consume them. and call of the wild. using the latest technology to keep people and bears a safe distance apart.

this is "nbc nightly news" with lester holt. >> good evening. another american has been brutally murdered by isis thugs. the awful news of the beheading of aid worker peter kassig came in a graphic isis video released earlier today. we won't show you even a frame on this broadcast, but we can tell you like all of the others, it demonstrates the sheer lethalness of the group. this is over a year after he was taken prison. tonight from the president on down, there are expressions of condemnation and from his family, the hope their son will be remembered for the good he was trying to do. richard engel reports tonight from turkey. richard? >> reporter: good evening, lester. unfortunately this very graphic 16 minute video has been authenticated, making-the fifth western hostage executed by isis since august, and the third american. peter kassig made it his life's

work to help others but paid for that selflessness with his own life. today the president called it an act of pure evil by a terrorist group that the world rightly associated with inhumanity. the army ranger from indiana traveling to iraq last year to volunteer as a medic. while delivering supplies, isis stopped him at a check point and kidnapped. he changed his name to abdul rahman, and his cell mates were taken one by one to be murdered. he knew his time would come. his father read a letter from his son. >> don't worry, dad, if i go down, i won't be thinking anything that i know it be true, that you and mom love me more than the moon and the stars.

>> kassig shared an audio recording of him before he was taken captive. >> i was able to share a little bit of hope and comfort for people. >> in a statement today the family asked for privacy and for the media not to show its murder, to deny the group what it wants, to terrorize. >> they've been forced to go out there and make appeals to isis leaders to free their lover lov ones. all of these have fallen on deaf ears. >> isis has proven time and time again its brutality. now this war shows no sign of ending soon. >> unlike other isis put out of hostages, this did not end with a threat to kill another host e hostage.

they are just splim running out of -- simply running out of hostages left to kill. this latest act of terrorism finds the president and leading officials are considering this. >> in a statement from the air force one the president said isis is bent on death and destruction. but the question for the strategy to defeat isis is working. >> in australia today, before learning of kassig's death, president obama said there are circumstances where the u.s. might need to send ground troops to fight isis. offering this doom's day scenario. >> if we discover that isil has gotten possession of a nuclear weapon and we had to run an operation to get it out of their hands, then, yes, i would order it. >> the remarks appear to show a slight change from his position two months ago. >> i will not commit you and the

rest of our armed forces to fighting another ground war in iraq. >> reporter: in washington last week, top u.s. general martin dempsey said he was open to using american ground troops to support iraqi security forces. >> i'm not predicting at this point that i would recommend that those forces in mosul but we'll consider it. >> dempsey said the tide is starting to turn in the battle against isil but acknowledged it will take years. the air strike is not a top force, but instead a bunch of midget running around with radical ideology. >> i think the american people feel it would be a serious mistake to make a commitment of land troops into the theaters. >> but the president's critics say it shouldn't be off the table. >> i think the president is wrong is saying that under no circumstances will he consider ground troops.

>> this month the president authorized doubling the number of troops in iraq to 3000, assigned to train and advise iraqi and kurdish forces. 900 airstrikes have slowed isis. >> airstrikes alone are not enough. you neat people on the ground -- need people on the ground to hold the campaign. >> and the people on the ground say the militia have infiltrated the iraqi military and proving how difficult it would be to depend on the iraqi army to fight isis. >> thank you. the area around houston, texas is back to normal after a chemical leak left four dead and released a cloud of gas. >> reporter: tonight a investigation no a deadly chemical leak is under way. it happened on saturday at this

dupont plant in la porte. two brothers were killed and the father writing on facebook, my sons were hard-working men and they lost their lives trying to help others. they will be missed but never forgotten. >> the whole neighborhood is so sad. we found out yesterday we lost someone we loved. >> a fifth worker was rushed to the hospital. the chemical is methyl mercaptan and has a strong rotten egg smell. it spread across the houston, metropolitan area, some 40 miles away. >> we thought something died in the house and started to burn candles, but it didn't go away. >> the company insists there was never a danger. >> there was never a hazard to our neighbors, but there was a strong odor and we deeply apologize for that. >> previously the u.s. chemical safety board found four

accidents at dupont facilities, including fatal ones in belle, west virginia and one in new york. >> one accident like this are one too much and that is why there are strict regulations on the books regulating methyl mercaptan and other chemicals used at this plant. are the regulations strict enough, were they enforced? how did the company respond to the regulation. >> they now have a team in texas to answer those questions and find out what went wrong. gabe guiterrez. the latest doctor arriving with ebola is in extremely critical. he may be the sickest person yet to be treated here. our report tonight from hallie jackson. >> reporter: medical workers carefully moved dr. martin salia

in a pod when he landed in omaha. the surgeon too sick to walk. >> he is seriously ill. more ill than any of the patients that have been transported from western africa to the united states. >> he was diagnosed with the ebola virus in sierra leone on monday. he was working in a hospital in free town. his son said he saw his father a month ago in maryland before he returned to west africa to help treat ebola patients. >> he decided to work there because he found out that was his true calling, he is a hero to me. >> today his wife appears to visit him. they hope to use drugs to treat the patient. he is the third patient to be treated at the bio containment unit and the fifth so far, including thomas eric dungeon. the ebola outbreak, the worst in history, has killed more than

5,000 people worldwide and nearly all of them in west africa. he explained why he needed to be there. >> he told me he needed to do it and i knew that deep within myself that the people in freetown needed help. >> the doctor who cared for so many, now the patient fighting for his life. hallie jackson, nbc news. with prayers and protests today, people in ferguson, missouri, marked 100 days since the police killing of an unarmed teen-ager who was black as the city is on eighth -- as the city is waiting for the return of the grand jury on the indictment. >> despite the frigid weather, they marked 100 days since michael brown was shot and killed. briefly blocking streets, some peacefully laying on the ground,

mimicking his death. >> it has been a long time now. haven't you made your point? >> no. the point won't be made until the cop is indicted. >> reporter: emotions are raw with a grand jury decision expected any day on whether officer darren wilson about be charged. surveillance video has been showing wilson in a police station two hours after the shooting and revealed police radio calls from that day, officers tracking a robbery suspect. >> 25, it's a black while with a white t-shirt. >> officers ask if he needs help. >> later wilson reports his location. >> 21, put me on canfield with two. >> according to the dispatch, that is wilson after he stopped michael brown and a friend. some 90 seconds later brown was dead, reports the local newspaper, based on interviews. another officer calls for help. >> get us several more units over here. there is going to be a problem.

>> we all have to work together more than ever now to make sure our community stays hole. >> reporter: the state highway patrol captain is visiting schools asking students to avoid trouble and promising police will be less confrontational when the grand jury reacts. >> what can we expect? >> i think they'll be a softer approach. >> and the protesters made their point. something people are hoping for when the grand jury indictment comes. this severe dose of winter weather returns with record cold temperatures and a lot of snow in the forecast. a real taste in kansas city, missouri, where mike seidel took this video of a frozen mountain. for more on what we can expect, let's turn to his weather channel colleague kim cunningham. >> good evening, lester. we are watching the front push east and bring snowfall. over two dozen states under

winter weather advisories right now. and also this is a big weather-maker as well. and this stationary boundary producing severe weather tonight in parts of louisiana. now this is the amount of snow we'll see through the ohio valley, 5-8 inches, including in cincinnati. expect travel problems tomorrow. but once the second front comes in, we'll see over 100 record lows. tuesday morning, look at this map for the ohio valley. we are talking 13 degrees and ft. wayne. this doesn't end. we see it across the east coast. record lows from the south all the way into new york city. get ready, another arctic blast is on the way. >> and that frozen mountain looked like a frozen fountain. when we return, keeping the popular energy drinks away from young children. new caution being urged tonight. and later, keeping the bears new caution being urged tonight. >[ male announcer ] we all think about life insurance.

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on a refrigerator. and the consequences can be serious. according to the study, some of the major effects of the energy drink exposure included cardiovascular events and neurological events, including seizures. we asked for comment and they say the poison control centers research have not added up. children under 12 have virtually no caffeine consumption from energy drinks but their organization which represents most of the energy drink companies in the u.s., agreed to voluntarily guidelines early in the year. they include labelling the drinks as not intended or recommended for children, no marketing to children under 12 and no energy drinks in schools. but again, news labels are voluntary. still today's report is a reminder to adults to keep track of their energy drinks and make sure they are not in places where children, especially small children, can get them.

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takes the reins in chicago, one of the largest and most important catholic archdiocese in america. >> i don't know how this developed. i don't have an explanation. >> he is the first in a century to lead the chicago 2.3 million catholics before he is an archbishop. >> it was unlikely that he would be chosen for chicago in 2014 as it was that barack obama would be elected president in 2008. >> while his predecessor is seen as a combative, the pope likes the more con silliatory message. >> talk and listen to people and make sure you really have an openness to where people are coming from. >> he's taken on his fellow bishops to failing to protect children from clergy accused of misconduct. and while he opposes same-sex

marriage and abortion, but encourages respect for gay people and prioritizing caring for the poor. one of the nine children who grew up poor in omaha. he chooses to live humbly. he drives a 2010 toyota camera and lives not in the bishop's residence, but in the seminary, sometimes doing the cooking himself. >> what is your specialty. >> i like to do different past yows or like a beef stew. some people like my mallenta. >> he said pope francis is leading the church to focus, like jesus, on serving the needy. >> someone said it this way. john paul 1i told us what to do. benedi benedict 16th told us why we should do it. and pope francis said to do it. >> the bishop said this is just getting started. >> as you say in television

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finally tonight 4 million people pay a visit to yosemite national park in california each year but for some the call of the wild can lead to frighteningen counters as the bears in the parks search for food. and that led scientists with a new approach to keep the bears away from the visitors. here is joe fryer. >> these days a visit to yosemite national park comes with a warning. >> a bear had gotten on to two of the cars and there were footprints on the cars so they were around the campsite. >> this shows what can happen when park visitors leave food in their cars. such encounters have plunged in the last 18 years, but this year there is a lit uptick. some blame the drought for the supply of the hundreds of bears that call yosemite a home. now the park has a new tool to

learn about their movements and find food. >> this is the american black bear we're tracking. >> wildlife biologists put sophisticated gps tracking collars on nine bears. >> see if we can locate the animal and get it out of the developed area. >> when night falls they find this 8-year-old male. >> we are about 50 yards from a bunch of people sleeping. >> the bear is clearly agitated. eventually the biologists back off. after some time passes -- they use gps to find him in a different spot and scare him. >> get out of there. >> once again, the bear takes shelter in a tree. this time he's farther from the campsite so the team feels comfortable letting him be. the next day gps tracking shows the bear had moved deeper into the wilderness. park rangers say the technology

is helping. >> gps collars have enabled us to better handle the bear situation, to help us achieve our ultimate goal which is keeping yosemite bears wild. >> for campers it is extra comfort so they can be close to nature, just not too close. joe fryer, nbc news, yosemite national park. that is "nbc nightly news" for this sunday. brian williams will be here tomorrow. up next, football night in america followed by the patriots versus the colts. i'm lester holt reporting from new york, for all of us here at nbc news, good night.

captions paid for by nbc-universal television tonight in indianapolis, "sunday night football" features a matchup of the two of the afc's best as the colts host the patriots. at present, andrew luck is actually on pace to narrowly top his predecessor peyton manning's nfl record for most passing yards in a single season. after beating manning and the broncos two weeks ago, bill belichick's patriots come in tonight looking to maintain their perch atop the afc.