A 37-year-old man has been arrested on suspicion of slipping behind an armed, contract RTD security officer wearing a uniform similar in style to Denver police late Tuesday before fatally shooting him in the head and running off.

Investigators say the attack, which happened just outside of Union Station, appears to be random.

Scott Von Lanken, a 56-year-old married father from Loveland, has been identified as the security guard. He was a former police officer in La Crosse, Wis., and for nearly 30 years was a pastor at charismatic Christian churches in Arizona, Ohio, Texas, and most recently, Loveland.

He had been employed by Allied Universal for six years.

A warning went out to all Denver police officers because there is no known motive for the shooting, which happened about 11 p.m. in Wynkoop Plaza, near the intersection of 16th and Wynkoop streets, police Chief Robert White said.

“We have concerns,” Denver police Cmdr. Barb Archer said. “Was the officer a target because he was wearing a uniform?”

The suspect, Joshua Cummings, was arrested a few blocks away and is being held without bond at Denver’s downtown jail for investigation of first-degree murder. A man with his same name and date of birth served in the Army from July 1997 to December 2002 on active duty.

Army records show Cummings, whose last station was Schweinfurt, Germany, was an infantryman and did not deploy. His last rank was sergeant.

Von Lanken was giving directions to two women about where to board the RTD light rail when the suspect crept up behind him and pointed a gun at his neck, Archer said.

The women, who had just left a nightclub, said they heard Cummings say “something to the effect of, ‘Do as I tell you,’ and then he shot the officer,” Archer said.

One of the women told investigators the suspect had “a swollen face and different/weird-looking eyes,” according to court documents.

Von Lanken died in an ambulance on the way to a hospital, according to an RTD news release. RTD security officers are armed, but they do not have official arresting power, Archer said.

White said Denver police responded to the shooting scene within minutes. Officers quickly viewed camera footage from nearby stores and tracked the suspect down within a half-hour.

Police also recovered a gun from the suspect, she said.

According to a probable cause statement for Cummings, he was found with a 9mm handgun with a live round in its chamber. The weapon also had an eight-round capacity magazine loaded with seven rounds. The rounds in the gun were FC 9mm Luger ammunition.

Police say they found one spent shell casing at the shooting scene — that of a FC 9mm Luger round, the statement says.

Lisa Meiman, a spokeswoman for the Denver office of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said her agency is assisting Denver police by tracing the gun.

Archer said the shooting was unprovoked as far as detectives know. White, however, said that detectives will also investigate whether the suspect shot the officer for a personal reason.

White said there is no reason to believe Cummings had an accomplice.

“We did ask officers to be vigilant,” White said. “We’re not 100 percent sure what motivated this individual.”

The killing had an immediate and profound affect on RTD officers and employees.

“It’s affecting everybody, even though he wasn’t a police officer,” RTD police Chief John Tarbert said.

Von Lanken worked as a police officer in Wisconsin from 1978 to 1987, according to his Facebook page. He then began working as a pastor in 1988. He moved to Loveland and continued to preach in 2002.

He and his wife, Shellie, have twin, adult daughters.

“God began to speak to them about starting FFR Revival Network to mobilize hungry people in the region for spiritual breakthrough that would bring lasting change to the churches and territory of Colorado,” according to a news release from Apostolic ARM. “They are parents of beautiful twin daughters who both have powerful prophetic giftings operating in their lives.”

In 2006, Von Lanken, then a revival organizer and pastor of Valley Assembly of God in Loveland, was quoted in an article in the Denver Post.

“We are establishing a Kingdom of God on the Front Range of Colorado,” Von Lanken said. “God is looking to move his kingdom beyond the wall of his church.”

Update:#DPD investigators on scene at 16th & Wyncoop conducting active death investigation. — Denver Police Dept. (@DenverPolice) February 1, 2017

“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 Wednesday morning.

Allied provides about 250 contract security officers to RTD, Tarbert said.

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

In a statement, Allied Universal said it was “devastated” by Von Lanken’s death. “We offer our deepest condolences to his family, friends and loved ones. The company intends to make a contribution in honor of the family to show our unwavering support. We ask that you join with us in keeping our fallen comrade’s family in your thoughts and prayers.”

The killing also left colleagues shocked.

“I feel anger, sadness and confusion as to why such an exemplary human being was so cowardly gunned down while doing his job,” Cole Peterson, an Allied employee who patrols for RTD in the Civic Center area, wrote in a post on Facebook. He gave The Denver Post permission to quote from his post.

“Officer Von Lanken was the consummate professional and one hell of a nice guy and he will be sorely missed by anyone who was lucky enough to cross paths with him,” Peterson said.