Devon Arthurs, 18, told police he had until recently shared his roommates' neo-Nazi beliefs but that he converted to Islam, according to court documents and a statement the Tampa Police Department released Monday.
Arthurs is being charged with two counts of first-degree murder, two counts of aggravated assault with intent to commit a felony and three counts of armed kidnapping. Court records did not list an attorney for him.
In the Tampa apartment with the victims' bodies Friday, investigators found Nazi and white supremacist propaganda; a framed picture of Oklahoma City bomber Timothy McVeigh; and explosives and radioactive substances, according to the court documents.
They also found a fourth roommate, Brandon Russell, crying and standing outside the apartment's front door in his US Army uniform.
"That's my roommate (Russell). He doesn't know what's going on and just found them like you guys did," Arthurs told the police officers, according to the report.
Federal agents arrested Russell, 21, on Saturday on charges related to the explosives.
The FBI said Russell "admitted to his neo-Nazi beliefs" and said he was a member of a group called Atomwaffen, which is German for "atomic weapon."
Arthurs started the chain of events Friday when he held two customers and an employee hostage at gunpoint at a Tampa smoke shop, police said. He was complaining about the treatment of Muslims.
"He further informed all three victims that he was upset due to America bombing his Muslim countries," police Detective Kenneth Nightlinger wrote in his report.
Officers talked Arthurs into letting the hostages go and dropping his weapon, and took him into custody.
While in custody, police said Arthurs started talking about killing two people, and then he directed them to a condominium complex where the four roommates shared an apartment.
"I had to do it," Arthurs told police.
Inside the apartment, the officers found the bodies of 22-year-old Jeremy Himmelman and 18-year-old Andrew Oneschuk. Both had been shot.
Police called in the FBI and a bomb squad, which found enough explosives to constitute a bomb, according to federal agents.
At first, Russell told agents that he kept the explosives from his days in an engineering club at the University of South Florida in 2013 and he used the substances to boost homemade rockets. The agents wrote that the substance found was "too energetic and volatile for these types of uses."
Russell has been charged with possession of an unregistered destructive device and unlawful storage of explosive material. Court records did not list an attorney for him.