 -- Tampa police are investigating after a man allegedly admitted to killing two of his roommates because he believed they disrespected his Muslim faith.

Devon Arthurs, 18, was arrested Friday and faces two counts of first-degree murder, two counts of aggravated assault and three counts of armed kidnapping.

Around 5:30 p.m. local time on May 19, Arthurs entered Green Planet Smoke Shop in Tampa armed with a black semiautomatic pistol, according to a police report. He allegedly demanded that the employee and one customer who were in the store get on the ground as he pointed the gun at one of the captives.

"Why shouldn't I kill you?" one victim said Arthurs yelled, according to the police report.

Two to three minutes after Arthurs entered the shop, a second customer entered and was also ordered to get on the ground.

"Arthurs informed all three victims in the store that he had already killed somebody," said the police report. "He further informed all three victims that he was upset due to America bombing his Muslim countries."

Approximately five minutes after the second customer entered the store, two Tampa police officers arrived at the scene and confronted Arthurs.

According to the police report, one victim was able to run from the scene, and the officers persuaded Arthurs to let the remaining two victims go. After minutes of negotiating, he surrendered and allowed officers to arrest him.

While being walked to a police car, he made references to "Allah Mohammed" and stated, "I had to do it. This wouldn't have had to happen if your country didn't bomb my country," according to the police report.

Asked if anyone else was hurt, Arthurs replied, "The people in the apartment, but they aren't hurt. They're dead," according to the police report.

He then directed police to his apartment, where two men were found dead. He identified the victims to police as his roommates Jeremy Himmelman, 22, and Andrew Oneschuk, 18, and provided details of the shooting — including the rifle he used, the sequence of events, exact locations and the placement of shots on each victim — according to the police report.

Arthurs said that he had once shared a common neo-Nazi belief with the two men before converting to Islam and that the killings stemmed from his roommates' disrespecting his Muslim faith.

The FBI arrested Arthurs' third roommate, Brandon Russell, on May 21 after allegedly being linked to explosive devices found in the apartment.

When police arrived at the apartment, Russell was seen outside the apartment dressed in full Army camouflage, "crying and visibly upset," according to the police report.

"That's my roommate," Arthurs said, according to the report. "He doesn't know what's going on and just found them like you guys did."

Arthurs told police that before the murders, he was aware of "Russell participating in online neo-Nazi internet chat rooms where he threatened to kill people and bomb infrastructure," according to the report.

"From our point of view, there was a double homicide, and we arrested the guy who did the homicide," a spokesperson for Tampa Police Department told ABC News. "When we started talking to him and got info about neo-Nazi and stuff in the apartment, we called the FBI, and we are certainly working with [it]."

Tampa police obtained a state search warrant for the apartment and discovered a cooler in the garage with a white cakelike substance. Two FBI and Tampa Police Department bomb squad officials identified the substance as hexamethylene triperoxide diamine, an explosive, according to a criminal report from the United States District Court for the Middle District of Florida.

Other bomb-making materials and firearms were found in the apartment. Nazi and white-supremacist propaganda were discovered in Russell's bedroom, including a framed photo of Oklahoma City federal building bomber Timothy McVeigh.

Russell, who is associated with the Army National Guard, allegedly admitted to law enforcement officials that he was a national socialist, a neo-Nazi and a member of a group called Atom Waffen, which is German for atomic weapon, according to the criminal report. They say he admitted to owning the explosive devices in the garage and said they were used to make homemade rockets in 2013 when he was in an engineering club at the University of South Florida.

Russell was charged with possession of unregistered destructive device and unlawful storage of explosive material. It is unclear if he is represented by an attorney and has not yet had a court date scheduled.

Arthurs, who is represented by a public defender, has a hearing set for May 24 at 10 a.m. local time.