By Police1 Staff

DENVER — A contract transit security officer is dead after a suspect approached him from behind and shot him in the neck.

Police told The Denver Channel that suspect Joshua Cumming, 37, fatally shot Officer Scott Von Lanken, 56, at Union Station Tuesday night.

An investigation found two witnesses were talking to Von Lanken when Cummings attacked. Denver Police Commander Barb Archer told The Denver Post the witnesses heard Cummings say “something to the effect of, ‘Do as I tell you,’ and then he shot the officer.”

According to an RTD news release, Von Lanken died on his way to the hospital. Von Lanken was a contract RTD security officer through Allied Universal. He was armed, but did not have official arresting powers.

The Denver Post reported Cummings was arrested within 20 minutes near the crime scene. He is currently being held for investigation of first-degree murder. Police recovered a gun from the suspect.

Cummings has an out-of-state criminal history and is on the federal terror watch list. Further information regarding why he is on the list was not available.

Authorities sent out a warning to officers after the shooting asking them to be vigilant because they don’t know the motive behind the shooting, the Associated Press reported.

Detectives said, as far as they know, the shooting was unprovoked. They are currently investigating if Cummings shot Von Lanken for a personal reason.

Von Lanken is survived by his wife and twin daughters.