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DENVER -- A group of Muslims in Colorado warned federal law enforcement officials that Joshua Cummings, the accused shooter of an RTD contract security officer, was "pretty advanced on his path of radicalization" and was "not listening to reason."

Investigators believe Cummings, 37, targeted RTD security officer Scott Von Lanken in what is being described as an execution-style killing.

Von Lanken, 56, was shot just after 11 p.m. Tuesday at 16th and Wynkoop streets on Union Station Plaza, one block from Union Station, the Denver Police Department said.

Police said two women were asking Von Lanken about light rail routes when a man approached from behind, pointed a gun at the officer’s neck and fired.

Law enforcement sources said Cummings had jihadist materials inside his backpack when he was arrested.

An email was sent to the Department of Homeland Security on Christmas Eve. It says a man who was only identified as "Joshua" attended a lunch for people who have converted to Islam.

The email warns that Joshua "feels that it [is] okay to fight now (not jihad/struggle, but actually fight), here to establish the rule of Islam."

Joshua incorrectly cited a verse from the Quran, claiming the Prophet Musa was asked to kill his entire community, the author of the email wrote.

Joshua had also "publicly rebuked" the presenter at a previous event for being "soft" and "not supporting enough the Hudud laws (criminal laws in shariah,)" according to the email.

"After talking to him for some time, he agreed to meet with some Imams to clarify his thoughts," the author wrote. "I am hoping to arrange a meeting of of of the stronger/more knowledgeable Imams with him to see if he can be mellowed a bit. But I doubt it would help. He is not listening to reason."

"You can't arrest someone for making radical statements," a retired FBI agent said.

Cummings’ friend Timothy said he grew up with the accused shooter and that Cummings had a normal childhood.

Timothy said he ran into Cummings a few months ago. During their conversation, Timothy said Cummings was high-strung and said he felt discriminated in the community because he was Muslim.

Timothy said Cummings went on to say that he was leaving and moving from Austin, Texas, to Colorado because he felt people were trying to force him out.

Cummings has an advisement hearing Friday.

Von Lanken was a former pastor at a church and left behind a wife and two daughters.

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