news 4 jackie benson has more on how the officer managed to rescue him, jackie? >> reporter: jim, we learned it ended when the 15-year-old victim was able to mouth the words "help me" to the police officer. detectives say these are the men the 15-year-old found when he showed up at the shopping center on little river turnpike in annandale last night. police say the teen arranged to meet there to sell the sneakers he advertised on line, according to sources. instead, after the teen got in the car, the men robbed him at gunpoint. lucky for him the vehicle caught the eye of the fairfax county police officer on patrol in the area. >> he saw a vehicle without its lights on slowly creeping across the parking lot. so he went ahead, did a stop. and there was a teenager, a young man in that vehicle who motioned some sort of distress signal. >> reporter: under arrest, penn and hall, and bryant robinson, 19, all four live in maryland. people who live near the shopping center say the location for the crime is surprising to them. >> very safe here. i shop here fairly often. i

news 4's jackie benson is live in annandale with a warning from police after all of this. >> reporter: wendy, police say the teen did the right thing by meeting the prospective sneaker buyer in a parking lot here. but they say he probably shouldn't have done so alone and at night. detectives say this 15-year-old found these men here, the 15-year-old had arranged to meet them here to sell the sneakers he advertised on line, on craigslist, according to sources. instead after the teen got in the car, they robbed him at gunpoint. luckily for him, the police officer was on patrol in the area and saw them. >> he saw the vehicle without its lights slowly creeping across the parking lot. and so he went ahead, did a stop. there was a teen. a young man in that vehicle who motioned some sort of distress signal to the officer. >> reporter: under arrest, marion kenny your, cypenn, and hall, and robinson, 19, all four live in maryland. police warn anyone who sells on line to take precautions like meeting in a public place, and bring a friend. >> make sure adults know where you're going. >> reporter

jackie benson is at lafayette park right across from the white house. what are you seeing there, jackie? >> reporter: i can tell you that heightened state of alert is very evident. we have some video that was shot a short time ago by an nbc news crew at the white house. what it shows is that shortly after news of the incident in boston, secret service agents sprang into action here at the white house. they very quickly put up yellow tape and issued orders to evacuate the plaza on pennsylvania avenue in front of the white house. it is my understanding that they did this with a great sense of urgency. >> the secret service came out. they ordered everyone out of the street and then they frantic kael started running up and down the street with the tape and closed everything off. i asked if it had anything to do with the explosions at the boston marathon. they said they don't know, they just got a radio call to do the evacuations and move everyone out of pennsylvania. >> this scene is being reported on capitol hill also, as i understand, it is a chain reactio