One thing I think is safe to assume: It was more than a one person attack. I read that these pressure cooker bombs weigh as much as 30 lbs. It is unlikely for one person to carry both at a time while being able to navigate through the crowds. Also, they could not watch over both bags before the detonation. They would have had to place one bag, retrieve another from their "safe" spot, and then place it, before sending a signal to detonate both. It sounds to me like there were at least two people planting the bombs. (Maybe others have different views).
To me the attacks were obviously well planned and premeditated. But they were not as devastating as they could have been (Did anything malfunction as far as we know?). In particular, I haven't heard anything about the 2nd bomb (further from the finish line). All of the casualties I have heard about (and pictures) have been from the finish line, probably because more cameras/people were there. I am just curious because it doesn't seem like a professional attack- the perpetrators obviously knew what they were doing, but I don't think they are "full-time" terrorists. So, I am more inclined to think it was a domestic attack, but who knows. I'm dumb and may have (probably) misanalysed many things.
Also mods, if this is the wrong place; feel free to delete/ edit.