Did Tamerlan and Dzhokar Tsarnaev buy their 9 mm Ruger legally?

The news media up to now has been unable to settle on a consistent story. Apparently reporters have just been guessing, or listening to authorities who were just guessing. The Tsarnaevs could not have legally bought a firearm in Massachusetts, a state with strict gun safety laws. Dzhokhar was too young, and Tamerlan did not have the required ID, nor was the weapon registered as required by Massachusetts law. Also, due to a domestic violence charge against him, he should have been unable to pass a criminal background check in his home state.

But it is a short drive to get to New Hampshire, where gun laws are next to nonexistent. We know the Boston Bombers drove to New Hampshire to buy the mortar style fireworks they disassembled to obtain the black powder for their bombs. Such fireworks can’t legally be sold in Massachusetts either. Tamerlan could have certainly legally purchased a firearm in New Hampshire even though he was on the terror watch list and had a history of criminal violence. How did he get the gun?