Bombs Go Off at Finish Line of Boston Marathon

[Angrboda]

Some news reports are still asserting that a person of interest is being guarded in a hospital with severe burns...who just happens to be a Saudi national here on a student visa...

I believe that guy was questioned and cleared by the FBI. Given that the explosives are thought to be either on a timer or activated remotely, it would be pretty stupid for the attacker(s) to stand in the vicinity when they went off.

Yeah, well not everybody is a Ted Kaczynski, and many bombers and other violent criminals tend to have an emotional bond with their crimes which results in strangely anomalous behavior. Many such people will stick around for the thrill of watching what happens. Criminal behavior can be hard to predict based on what a "sane, rational person" would do.