What I gather is the FBI was convinced by the Killer that the complaints of his Coworkers were based on "Anti-Muslim" Bigotry. Also the DHS investigation may have been shutdown because it was perceived as being Anti-Muslim.

Hell this murderous bastard worked for a security company that has contracts with the DHS. Although in this case the Killer is reported to have been working as a guard at a gated resdential community.

Will we every know for certain ? Probably not as it seems our Federal Agencies lie as easily as they breathe.

Mine's a tale that can't be told, my freedom I hold dear.