Denver Mosque Warned Feds About Murder Suspect's Alleged Radicalization

Joshua Cummings is suspected in the fatal shooting of a Denver transit officer.

— -- The suspect in the alleged execution-style killing of a Denver transit officer was reported to federal officials by members of a local mosque who were alarmed at his apparent radicalization, according to an email obtained by ABC News.

DHS said in a statement today it “did receive the e-mail in question from a community member in Denver. It was immediately referred to the appropriate law enforcement agencies for review.”

Police say Cummings, an Army veteran and Muslim convert, approached a uniformed Denver transit security officer who was helping two women with directions Tuesday and allegedly opened fire into his neck, killing the officer.

He told the officer, "Do as I tell you," before allegedly pulling the trigger, police say. The officer has been identified as 56-year-old Scott Von Lanken of Loveland, Colorado.

Police arrested Cummings Tuesday a few blocks away and allegedly recovered a handgun.

"He seems pretty advanced in his path of radicalization," the email warned DHS. "He also feels that it [is] okay to fight now (not jihad/struggle, but actually fight), here to establish the rule of Islam."

The email said that representatives from the mosque had arranged for Cummings to meet with "knowledgeable Imams," to "see if he can be mellowed a bit."

"But I doubt it would help," the letter concludes. "He is not listening to reason."

ABC News' Clayton Sandell contributed to this report.