The gunman accused of shooting and killing a Denver transit security official on Tuesday is being described by law enforcement sources as a 'radical Muslim.'

Joshua Cummings, 37, was arrested late Tuesday by police just 20 minutes after he allegedly shot dead Scott Von Lanken, 56, a security contractor working for Denver's Regional Transportation District.

Two unnamed sources close to the investigation say Cummings had 'jihadist materials' in his possession, according to Fox 31 TV.

He also had a handgun and ammunition on him.

Cummings is a former US serviceman who was kicked out of a Texas mosque, according to KUSA-TV.

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Joshua Cummings (left), 37, was arrested late Tuesday by police just 20 minutes after he allegedly shot dead Scott Von Lanken (right), 56

Von Lanken was talking to two women trying to catch a late night train home when he was shot and killed.

Cummings alllegedly came up from behind, stuck a handgun to the officer's neck and fired, according to Denver police.

The shooting happened late Tuesday night near Union Station, a hub for buses and trains, and the city's pedestrian mall.

Security camera footage helped police quickly find and arrest Cummings.

Von Lanken (right) was talking to two women trying to catch a late night train home when he was shot and killed

Von Lanken was wearing a dark blue uniform similar to those worn by police.

In case he was targeted because he was believed to be a police officer, police Chief Robert White said officers have been warned to remain vigilant.

According to police, Von Lanken was trying to help two women who were afraid they had missed the last light rail train when one of them said she saw a man with a swollen face and 'weird looking eyes' walk up to the officer and say something to the effect of 'Do what you are told' before she heard a gunshot.

He ran away but police found Cummings hiding on the patio of a nearby loft apartment building with a 9mm handgun.

Cummings, who has ties to a variety of cities in Texas, most recently Austin, was charged with a misdemeanor over five years ago out of state, police Commander Barb Archer said.

Cummings alllegedly came up from behind, stuck a handgun to the officer's neck and fired, according to Denver police. Above is a photo of the crime scene after the attack

On social media, Cummings identified himself as a jiu-jitsu instructor.

He was particularly active on Twitter in 2016.

'I seek refuge in the one God from the evils of Satan, and I begin in the Name of Allah, the Most Merciful, the …' he wrote in June 25, 2016.

'Wake up people. America is a failed state. It's about to get so real. …' he posted on June 26, 2016.

It's not clear if he has a lawyer yet.

Gary Kim, the manager of the Holiday Motel in the Denver suburb of Englewood, said Cummings had been staying there for about three weeks.

Cummings previously stayed for about a month at the $365-a-week motel before leaving in late November and then returned in early January.

'I'm just pretty blown away. ... He was one of my favorite tenants. I enjoyed seeing him,' Kim said.

The motel manager said he didn't know what Cummings did for a living, but he would often volunteer to help people pay their rent.

Kim added that Cummings 'kinda looked like a hippy' and had a full beard.

A makeshift memorial put up by Denver Police is affixed to a pole early Wednesday near the scene where Von Lanken was shot and killed late Tuesday

He stayed at the hotel with a woman and a child, and Kim said he never noticed anything out of the ordinary.

Von Lanken was a contracted security officer for the Denver area's Regional Transportation District employed by Allied Universal.

Shellie Von Lanken told KUSA-TV in Denver that her husband of 35 years worked at least 65 hours a week to support her and their 32-year-old twin daughters, one of whom is disabled.

'It was unbelievable that any human being could even work what he was working,' she said.

'He just worked his heart out. He would tell me, 'If I could keep working, I would get another job just so I could provide for my family'.'

She added that if her husband were still alive, he would tell her and their daughters to forgive the shooter.