"It's a criminal domestic terrorism investigation," Turgal told reporter Brahm Resnik of television station KPNX, "and we define that as individuals or oganizations who are engaging or conspiring to engage in criminal activity in order to affect either social or political change."
Authorities said Ready walked into a house in a suburb east of Phoenix on Wednesday and opened fire, killing four people, including his girlfriend and a toddler, before killing himself. At the scene of the massacre, investigators said they discovered at least six projectile grenades and other explosives.
Ready was a longtime white supremacist who in recent years formed a group he called the U.S. Border Guard, which routinely patrolled vast parts of the Arizona desert looking for illegal immigrants.
Turgal said investigators were never able to develop enough evidence to arrest Ready, but he said the investigation was continuing even after the white supremacist's death. He said the FBI was still looking into the surviving members of the US Border Guard and other organizations Ready was associated with.
KPNX also obtained a search warrant affidavit that revealed conflicting stories about the massacre which raised the possibility that a sixth person might have left the house after the killing. It was unclear whether that possibility was credible.
Watch the KPNX report here: