That’s photo shopped
It's legit.
And the photographer has spoken with a bunch of FBI. They now have the pic.
Here's the story verifying: NY Times
And here's a link to a very high-res copy of the photo: Photo by David Green, who had run in the marathon and was heading to meet friends, clicked the image on his Iphone and posted it on his FB page. He was unaware he caught the suspect, his FB friends noticed.
Read the article, very informative.