DENVER — A Denver mosque warned the U.S. Department of Homeland Security a Muslim convert was radicalized more than a month before he was arrested in the shooting death of a security guard, officials said on Thursday.

Joshua Cummings, 37, was taken into custody in Denver on Tuesday on suspicion of fatally shooting Scott Von Lanken, a transportation agency security guard and former pastor, that night, police said.

Detectives are investigating whether Von Lanken was targeted because he wore a uniform similar to those worn by police officers, Denver Police Chief Robert White told reporters.

Leaders of the unnamed mosque said in a Dec. 24 email that they were concerned Cummings was espousing radical beliefs when he attended meetings for converts to Islam.

Qusair Mohamedbhai, a Denver attorney, released the email on behalf of the mosque to the media on Thursday.

In a statement accompanying the email, Mohamedbhai said the local Muslim community “will always oppose those who undermine our collective American values.”

The author of the email, whose name is redacted, said Cummings espoused radical beliefs including using violence to establish Islamic law, and was not “listening to reason.”

“He seems pretty advanced in his path of radicalization,” the email said.

Homeland Security spokeswoman Gillian Christensen confirmed the department had received the missive.

“It was immediately referred to the appropriate law enforcement agencies for review,” Christensen said, without identifying who was notified.

A Denver television station aired video of FBI agents searching the motel where Cummings had been living.

The Federal Bureau of Investigation declined comment on the case.

It was not immediately clear if Cummings has an attorney.

Von Lanken, 56, was speaking with two women outside Denver’s Union Station transit depot when Cummings walked up to him saying, “Do what you were told” and shot him in the neck, according to an affidavit filed in court by Denver Police.

When police apprehended Cummings a short time after the shooting, he was carrying a loaded 9mm handgun, the police affidavit said.

Von Lanken, an ex-police officer, died from a single gunshot wound, the Denver Office of the Medical Examiner said in a statement.

Cummings, who is held in jail without bond, is due in Denver District Court on Friday for a hearing but it is unclear when he will be formally charged.