A Denver transit police officer who was wearing a dark blue uniform was ambushed and killed Tuesday night.

Scott Von Lanken, 56, was killed while working as a transit security officer with Allied Universal. Denver's RTD contracts with Allied Universal to provide security services, the Denver Channel reports.

Through investigation, police say they learned two witnesses were talking to Von Lanken before the shooting when a suspect approached from behind, pointed a gun at the victim's neck and fired. 

After taking the man into custody, police identified him as Joshua Cummings, a 37-year-old who has an out-of-state criminal history. Police in Denver have no history with the man. 

Cummings is from Texas and reportedly a convert to Islam with radical beliefs.

The Denver Muslim community raised flags to federal agents in late December that a convert named Joshua was “very radicalized,” according to an email obtained by Denver7.

Muslim leaders in Denver on Thursday told Denver7 Cummings had attended an event at an area mosque on Dec. 24 and another open house at the mosque earlier that month, and that several others at the meetings were concerned about his talk and behavior.

They were concerned enough that they sent a letter to the U.S. Department of Homeland Security that same day tipping federal agents off about Cummings’ behavior.