Authorities say the gunman was also killed after he attacked a group of deputies from Douglas County Sheriff.
A month before Matthew Riehl shot and killed a deputy and injured four others in Colorado, authorities were warned that Riehl was exhibiting signs of mental illness and could be a danger.
Police told of a suspect opening fire on deputies who'd arrived at an apartment complex in Highlands Ranch just on 5.00am to investigate a noise complaint, injuring four and killing Deputy Zachary Parrish.
A gunman killed a sheriff's deputy and wounded six others Sunday before dying in a hail of gunfire, a bloody scene that shook this quiet suburban community on the last day of the year.
Parrish, the slain officer, was 29 and a married father of two young children.
"I could hear 15 to 20 gunshots after I got off the phone with her", Silknitter told HuffPost.
A caller said Riehl was acting freakish and might be having a mental breakdown, but responding deputies found no evidence of a crime and left. "It was very hard today to go see Zach for the last time".
The incident occurred around 5.15am local time at Copper Canyon Apartments, a landscaped apartment complex in Highland Ranch, 16 miles south of Denver.
The investigation will likely also include interviews with the officers and any witnesses as well as the man who reported to police that he believed the suspect was suffering from a mental breakdown.
"One male said the suspect was acting freaky and might be having a mental breakdown", a police news release said.
He said: "I do know that all of them were shot very, very quickly". "They all went down".
"There were well over 100 rounds fired in this engagement". He described it as being like an ambush-type attack.
While many details remain unclear, Spurlock said Sunday that the department hoped that the officers' body cameras would help shed light on what happened during the deadly incident.
"He knew we were coming, and we obviously let him know that we were there", Spurlock said. The civilians who were shot, all survived.
When the officers enter Riehl's apartment, he continues the livestream.
He said the rifle-firing suspect didn't have any known rap sheet but "had encounters with law throughout [Denver] metro area".
The injured officers were identified as Mike Doyle, 28; Taylor Davis, 30; Jeffrey Pelle, 32, and Tom O'Donnell 31. The son of Boulder County Sheriff Joe Pelle was in fair condition.
"We are now with our son and his wife".
DCS holds a press conference where they release the names of the officers involved. It's also minutes from Columbine, where, in 1999, two teens gunned down 12 students and one teacher at that neighborhood's high school, and wounded 21 more students, before taking their own lives.
Spurlock said the injured civilians may have been bystanders in other apartments but cautioned he hadn't confirmed that yet.