Lakewood police spokesman Steve Davis said 35-year-old James Davies died "after being shot by another officer" in darkness outside a house where police had set up a perimeter and made arrests, The Denver Post reported.
Davies, who was married and the father of two children, had been on the force 6 1/2 years, Davis said.
The officer who fired the shot, not knowing his gun was aimed at a fellow officer on the other side of a fence, was not identified.
"Imagine the anguish the other officer is going through," Davis said. "We feel like we have two victims on the department."
Police had gone to a Lakewood neighborhood in the early morning hours to investigate reports of shots being fired and surrounded the house, Davis said.
Police said someone from inside shot at officers and they fired back.
"It went downhill from there," said Ty Countryman, another Lakewood police spokesman.
Police entered the house and brought out three male suspects.
As that happened, another shot was fired and the officer was hit, police said.
The 43-year-old Lakewood Police Department had not previously had an officer killed in the line of duty.
Officers from Denver, Edgewater and Jefferson County had joined Lakewood police at the shooting scene, which is in a neighborhood residents described as generally safe and quiet.