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DENVER -- The man suspected of shooting and killing a uniformed RTD contract security officer in Lower Downtown late Tuesday night has been described as a radical Muslim, law enforcement sources said Wednesday.

The suspect was identified as 37-year-old Joshua Cummings.

Two sources said investigators found jihadist materials in Cummings' backpack and those materials are being investigated.

Scott Von Lanken, 56, was shot and killed, the Denver Office of the Medical Examiner confirmed.

The shooting happened about 11 p.m. at 16th and Wynkoop streets on Union Station Plaza, one block from Union Station, police said.

"This is an incredibly sad day for us, and on behalf of the entire RTD family I extend my prayers and express my sincere condolences to the officer’s family, friends and loved ones," RTD general manager and CEO Dave Genova said in a statement.

Police said at a news conference Wednesday morning that two women were asking the armed security officer about light rail routes when a man approached from behind, pointed a gun at the officer's neck and fired.

Cummings was found hiding in the 1600 block of 14th Street about 20 minutes later and was arrested without incident, police said. A gun was also recovered.

The shooting is being investigated as a homicide and police are looking into whether it had to do with Von Lanken wearing a uniform.

"It's really disheartening when anyone loses their life, but especially when we have someone out there providing safety to the community at large," police chief Robert White said.

Police said Cummings lives in the area and have not had contact with him. He had an out-of-state misdemeanor arrest more than five years ago. White said the motivation for the shooting is not known.

Von Lanken was employed by Allied Universal, which provides security services under contract for RTD.

"Our entire team is deeply saddened by this loss," RTD assistant general manager of safety and security Mike Meader said. "RTD and Allied Universal have been in contact with the officer’s family and will continue to offer our support during this sad time."

Wynkoop was closed between 15th and 17th streets on Wednesday morning as police investigated the shooting. It reopened about 5:50 a.m.

Officers later set up a makeshift memorial at the scene with flowers and signs for the man. One sign said, "Prayers from the downtown motorcycle unit."

"Prayers from the downtown motorcycle unit." - Denver police officers create memorial for man who died in overnight shooting on 16th St. pic.twitter.com/9dNh0yzcDG — Amanda Zitzman (@AmandaZitzman) February 1, 2017

RTD has 250 uniformed Allied Barton security officers who patrol 24 hours a day, and about 265 contracted extra-duty police officers from Lakewood, Aurora and Denver. There are also eight internal transit police officers.

A memorial fund to help support Van Lanken's wife and two children has been established through the Rocky Mountain Law Enforcement Credit Union.

It's under Scott Van Lanken's name on the credit union's website or by calling 303-458-6660.

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