The investigation began unfolding Friday, when Arthurs held two customers and an employee hostage at gunpoint at a Tampa smoke shop, police said.
Arthurs said he converted to Islam and was upset about American bombings in Muslim countries, among other issues, according to a Tampa police report. He is being charged with two counts of first-degree murder and other charges, and court records did not list an attorney for him.
Officers talked Arthurs into letting the hostages go and dropping his weapon, and took him into custody, according to officials. 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.
When they arrived at the apartment the fourth roommate, Brandon Russell, was crying and standing outside the apartment's front door in his military uniform. He had just finished duty with the Florida National Guard.
Inside lay the bodies of 22-year-old Jeremy Himmelman and 18-year-old Andrew Oneschuk. Both had been shot.
Investigators also found a cooler filled with bomb-making material, two radioactive substances and the Nazi propaganda, according to the FBI. 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."
FBI criminal complaint document: