As of right now, no one knows anything, pretty much. The White House is making a connection to Patriots Day. I expect to hear that Sara Palin is wanted for questioning any time now.
That didn't take long.
It appears that the bombs themselves were pressure cooker bombs made from directions available on the Al-Quaida magazine "Inspire". These things are common in Pakistan where they get used in electioneering campaigns. The essential components are one 6-liter pressure cooker, 6 lb of smokeless powder generally extracted from unfired munitions which are common enough in the middle east, and whatever small bits of junk may be found lying about to be used as shrapnel.
In Israel, small nuts are popular, and certainly cheaper than ball bearings.
Smokeless powder for reloading and shotgun shells which contain larger quantities of powder per shell to hit the banned list in 3....2....1....
Update: Washington is banning trash cans.