TAMPA — A man accused of shooting his two roommates Friday in a Tampa Palms apartment told police he shared neo-Nazi beliefs with the men until he converted to Islam then killed them because they showed disrespect for his faith.

The revelations weren't over.

Officers found a garage stocked with bomb materials as they arrived to investigate the double homicide, leading to federal explosive charges against Brandon Russell — a Florida National Guardsman and admitted neo-Nazi who kept a framed photo of Oklahoma City bomber Timothy McVeigh on his dresser.

Russell, 21, was the man Tampa police officers found crying outside his door Friday evening when murder suspect Devon Arthurs led them back to the apartment that the four had shared in an affluent suburb north of the University of South Florida. Russell, wearing camouflage, had just returned from National Guard duties.

Police went to the apartment in the Hamptons at Tampa Palms after Arthurs, 18, told them he fatally shot his roommates Jeremy Himmelman, 22, and Andrew Oneschuk, 18, according to a Tampa police report.

While searching the garage, investigators found a cooler full of a white, cake-like explosive material known as HMTD, or hexamethylene tiperoxide diamine, according to a criminal complaint filed in U.S. District Court. Nearby, they found explosive precursors — chemicals that can be mixed to create explosives — including potassium chlorate, potassium nitrate, nitro methane and more than a pound of ammonium nitrate in a package addressed to Russell.

Investigators also found electric matches and empty 5.56-caliber ammunition casings with fuses that could be used to detonate destructive devices once HMTD was added to the casings. The materials could be used to make a bomb, according to the complaint.

In addition, pagers carried by bomb technicians alerted them to the presence of two radiation sources. The complaint does not say whether they were connected to the explosive materials.

In Russell's bedroom, investigators discovered Nazi and white supremacist propaganda including the photo of McVeigh, who was convicted and executed for detonating an ammonium nitrate and nitromethane fertilizer truck bomb in front of the Alfred P. Murrah Federal Building in Oklahoma City in 1995. The death toll from the blast was 168.

According to the complaint, Russell admitted to being a national socialist, manufacturing the HTMD and owning the precursors. He also admitted he is a member of a white supremacy group called the Atomwaffen, German for "atomic weapons."

Questioned about why he had the explosives, Russell said he was in a USF engineering club in 2013 and used the HMTD, in part, to boost homemade rockets and to send balloons into the atmosphere.

"Based on my training and experience, HMTD is too energetic and volatile for these types of uses," FBI Special Agent Timothy A. Swanson wrote in the complaint.

Arthurs told investigators that all four roommates shared neo-Nazi beliefs until he converted to Islam, according to the complaint.

"Arthurs stated that for some time before the murders, he had been privy to Russell participating in online neo-Nazi internet chat rooms where he threatened to kill people and bomb infrastructure."

Russell was arrested on a FBI warrant Sunday in Key Largo and charged with possessing an unregistered destructive device and unlawful storage of explosive material. It was unclear Monday what he had been doing since Friday, but the FBI said in an email to the Tampa Bay Times that Key Largo "is part of the active investigation and will not be commented on at this time."

It was also unclear whether the Florida National Guard will conduct its own investigation into Russell's actions or await the results of the FBI's, said Maj. Caitlin Brown, a guard spokeswoman in Jacksonville.

The Atomwaffen Division is a small, loose group of neo-Nazis that formed in the last couple of years, according to the Anti-Defamation League. Considered a hate group by the league and the Southern Poverty Law Center, the Atomwaffen Division circulates white supremacist fliers urging students to join local Nazis on college campuses around the country. Universities targeted include the University of Central Florida and State College of Florida Manatee-Sarasota.

USF police said there is no evidence that any white supremacist group has recruited on campus for the last several years.

Members of Atomwaffen congregate on a website that describes them as a "Global Fascist Fraternity" and urges "race war now!"

As news of the slayings began to spread, a chat thread emerged in which members mourned Himmelman and Oneschuk as "fallen Aryan brothers" and used racist epithets to describe Arthurs as a Muslim traitor.

In an interview Sunday, Himmelman's sister Lyssa said she was aware that Arthurs and Russell were part of a white supremacist group but said her brother and Oneschuk were not involved and did not agree with Arthurs' extreme views.

She said Arthurs had invited them to move in but that her brother and Oneschuk had planned to move out Monday "because of how extreme Devon was being." She described Jeremy as a "sweet, funny, amazing loving brother who would never hurt a fly."

Reached Monday, Lyssa Himmelman called Arthurs' claims that all the roommates were neo-Nazis "lies" and declined further comment.

Russell's arrest is the latest twist in a case that began unfolding Friday when Arthurs briefly took hostages at the Green Planet Smoke Shop down the street from his apartment. A Tampa police report obtained by the Times on Monday provides new details about the hostage-taking and killings.

Arthurs entered the smoke shop at 15325 Amberly Drive about 5:30 p.m. and pulled a semiautomatic pistol from his waistband.

"Do me a favor and get the f--- on the ground!" he yelled to a female employee and male customer, the report states. Arthurs asked the customer, "Why shouldn't I kill you?"

A few minutes later, another customer entered the store and Arthurs ordered him to get down. He told all three people in the store that he had already killed someone.

"He further informed all three victims that he was upset due to America bombing his Muslim countries," according to the report, by police Detective Kenneth Nightlinger.

Two Tampa police officers arrived about five minutes after the second customer entered the store. One hostage ran out of the store and the officers were able to persuade Arthurs to let the other two leave. After several more minutes of negotiating, Arthurs surrendered and allowed the officers to place him in handcuffs.

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Arthurs made references to "Allah Mohammed" as officers walked him to a patrol car.

"I had to do it," he said, according to the report. "This wouldn't have had to happen if your country didn't bomb my country."

Jail records show Arthurs was born in Florida.

Asked if anyone else was hurt, he told police: "The people in the apartment, but they aren't hurt, they're dead."

Arthurs directed police to unit No. 3723 in the Hamptons. There, police found Russell in his National Guard camouflage, standing just outside the door, "crying and visibly upset." Arthurs saw Russell and told police, "That's my roommate. He doesn't know what's going on and just found them like you guys did."

Officers entered the apartment and found Himmelman and Oneschuk inside. They'd been shot in the head and upper body, the report states.

In an interview with investigators, Arthurs provided details about the shooting, including the rifle he used, the order in which he killed the men and which part of their bodies he targeted.

Arthurs said that, before the killings, "he had been privy to neo-Nazi internet sites threatening to kill people, and he had developed a thinking that he should take some of the neo-Nazis with him."

Arthurs told police he had become angry about the world's anti-Muslim sentiment and "wanted to bring attention to his cause.

Arthurs faces two counts of first-degree murder, two counts of aggravated assault and three counts of armed kidnapping. He remained in the Hillsborough County jail without bail on Monday.

He is scheduled to appear in court at 10 a.m. Wednesday before Hillsborough County Judge Margaret Taylor.

Staff writers Dan Sullivan, Anastasia Dawson, Howard Altman and Colleen Wright contributed to this report. Contact Tony Marrero at tmarrero@tampabay.com or (813) 226-3374. Follow @tmarrerotimes.