Everyone in America knows what the Boston Marathon is although most don't celebrate it, most equate the marathon with Patriot's Day (even if they don't know what that means .) Do amateurs us ball barrings in their explosives?
After seeing it in 50 movies ... yeah probably. I think it was an amateur, they say the strength of the explosion was low considering the size of the bombs. Apparently a pro could have made the bomb about 5 times stronger ... at least that was what I heard on Fox news a few minutes ago. (I don't know shit about bombs so i'm just telling you what they said)