It's looking more and more like it was a lone nut.
As I write this, at 7pm on Wednesday, the authorities seem confident they have the person in question identified. There have been rumors all day that there had been an arrest or that they had someone in custody; the FBI has denied this, but the federal courthouse in Boston was evacuated due to a bomb threat, whether this was related is not known. But there has been a lot of speculation that the suspect was in the building.
There was supposed to be a press conference at 1pm, which was later pushed back to 5pm, which has since been pushed back again. Some sources have reported off the record that an operation is under way. I bet we see an arrest tonight or tomorrow morning.