ISIS does require all it's business transactions to be Muslim. That isn't hard to do.
Reports indicate that he made his millions in online gambling and real estate. I'm calling BS on that. Online gambling is one way in which illegal money is laundered. That's a fact. He would have taken the laundered money and then invested in real estate. Real estate is also a means by which illegal money is laundered incidentally.
So, this kinda points to him being a criminal. His father was a bank robber and on the FBI's most wanted list. The acorn doesn't fall far from the tree. another fact-he was an IRS agent and an accountant. He would definitely know how to hide illegal money from the government. I would also like to know when, exactly, this 64 year old man "retired". He seems to have retired earlier than most civil servant accountants.
I'm just looking outside of the box because inside the box doesn't make a lot of sense and there are conflicting stories about what actually happened inside the box e.g multiple shooters, a woman in the concert reportedly telling concert goers that they're going to die. There's the fact that the cops can't seem to figure out when he allegedly shot himself. Police claim that he had an escape plan set up and he wired a 100 grand to his girlfriend in the philipines? Sorry, that's suspicious. The Philipines happen to be an ISIS stronghold. He apparently converted to Islam 6 months ago?
Lone wolf? Maybe. But there's enough here to support a conspiracy theory IMO.