I think it was an arms deal gone bad.
Nobody has 40 guns and thousands of rounds of ammo with them in one place unless they're doing business. He was an informant for the FBI (hence the cameras in the room) maybe by choice or maybe by force. The deal either was known to be bad from the buyers going in or went bad in the process. The buyers (maybe some Muslim group) decided to turn the tables, killed the guy, smashed a few windows and opened fire to create a diversion/settle score/make political statement. The FBI had prior knowledge. The 60 minute time frame between end of shooting and cops coming in was for the FBI to grab the memory sticks from the cameras, arrange the body to look like a suicide, clean up their traces and hastily arrange "evidence".
This is why he had more guns and ammo in his trunk. Probably to sweeten the deal if needed. The shooting was clearing a military grade automatic regardless of what was "found" in the room". The girl who warned people was a hooker that had been staying with him and either got out early or was told to leave. The Philippines has been a center for American intelligence agencies to move drugs and guns for decades. The GF in the Philippines is that local contact.
That's where he made his real money, why he was always on the move and why he had two small planes. Also explains why they can't pin a political/religious motive on this and "magically" none of the cameras in the room recorded the incident.

Just my $0.02