Felony possession of a firearm in violation of 18 USC 922 (g) 8.
He had a restraining order issued against him by his ex-wife with language forbidding him from committing violence against his ex-wife and children.
The REAL question to ask is why the Brady check failed to prevent an unqualified person from purchasing a firearm under such circumstances?
Perhaps he bought it in a private transfer, or there was a straw purchase (also illegal) involved. But the timing appears to look like it went like this:
POS puts in order at dealer, dealer contacts distributor, distributor obtains firearm from Bushmaster in June. Distributor send firearm to dealer. Dealer lawfully delivers firearm after Brady check fails to prevent it. POS has obtained a firearm in violation of Federal law.