The FBI has video of the two now one suspects placing the bag in the location of the detonation. The man that had his legs blown off identified one of the two suspects right before the bomb went off and way before they released the photos when he went to the hospital.
The man who IDd one of the two is Jeff Bauman. Hes the guy in the photos of the guy in the wheel chair with both his legs blown off at the knees.
In the days after the attack, Baumans memory of the man in the cap might have been the critical clue that led investigators to the Tsarnaev brothers 19-year-old Dzhokhar and 26-year-old Tamerlan who are suspected of mounting the twin bombings that killed three and wounded more than 170.
Still in intensive care, Bauman woke up, under so much drugs, asked for a paper and pen and wrote, bag, saw the guy, looked right me, his brother, Chris, revealed yesterday.
http://www.independent.co.uk/news/world/americas/boston-marathon-bombing-how-critically-injured-jeff-baumans-memory-of-man-in-the-cap-gave-fbi-vital-clue-8580950.html
It sounds to me like Jeff Bauman didnt make the ID until a few days after the bombing because he was in pretty bad shape, in the ICU and heavily sedated.