The man authorities say killed one Colorado sheriff's deputy and wounded four others was an Iraq War veteran who allegedly made threats to the law school from which he graduated years earlier.
Matthew Riehl, 37, was identified by Douglas County Sheriff Tony Spurlock as the man who was initially causing a disturbance that summoned cops to his apartment two times on Sunday.
The first visit by authorities was at around 1:30 a.m. local time in response to a noise complaint. Four hours later, officials returned to Riehl's home for a domestic disturbance complaint, where they were shot "very, very quickly" in what the sheriff described as an "ambush-type of attack."
Douglas County Deputy Zackari Parrish, 29, was killed, Spurlock said. He is survived by his wife and two young children.
The wounded deputies -- Mike Doyle, 28; Taylor Davis, 30; Jeffrey Pelle, 32; and Castle Rock Police Officer Tom O'Donnell, 41 -- were said to be in stable condition.
The two civilians in neighboring apartments were shot during the standoff but the injuries were not considered life-threatening, Spurlock said.
Back on Nov. 6, Riehl, who served in the Wyoming National Guard and was deployed to Iraq in 2009 before being honorably discharged in 2012, was the subject of an email sent out by the University of Wyoming College of Law Assistant Dean Lindsay Hoyt with a subject line that read: "Suspicious Person."
In it, students were warned about receiving "information about suspicious behavior involving a former student," the email, acquired by ABC News, states. Students, staff and faculty were informed that there were "no direct threats" to any of them the school administration was notifying them as a "precaution."
Hoyt encouraged students to be on the lookout and "immediately notify" campus police "if they observe the former student or his red Ford 'Foxbody' Mustang with Colorado plates in the area of the Law School."
Photos of both Riehl and his Ford were attached to the email.
The email was provoked by “rambling, nonsensical messages" posted on Riehl's Facebook page, Chad Baldwin, a University of Wyoming spokesman, told ABC News.
He confirmed that Riehl, who authorities say died after engaging in Sunday's gunfight with officials, graduated from the law school in 2010.
Baldwin characterized the posts as "alarming in nature" and as a result, he said the university's police opened a probe and notified law enforcement in Colorado.
"The nature of those posts was alarming enough that there was concern for the safety of the law school," he said.
The Douglas County Sheriff's department began monitoring Riehl's YouTube page because he had been tagging them in his postings, officials said.
Parrish was a father with two children, and a "good kid, a smiley kid" who was "eager to work and eager to serve," Spurlock said Sunday.
"When I sat with his wife and held her hand I could see in her eyes that the life they planned together was over," he said.
"It was difficult to see Zack for the last time," he said. "He loved this job more than any job he ever had."
The officers who responded to the second call were let in by a male tenant who lived with Riehl, who had barricaded himself in a bedroom, Spurlock said. They attempted to talk to him, but he opened fire on them, Spurlock said.
UPDATE, we have multiple deputies down, no update on their status. The scene remains active and please avoid the area.— DC Sheriff (@dcsheriff) December 31, 2017
The sheriff said Riehl fired around 100 bullets.
He described the attack as an ambush because the suspect knew they were coming in to investigate the disturbance.
"It was more of an ambush-type attack on our officers," he said. "He knew we were coming and we obviously let him know we were there to investigate the disturbance."
The officers were all shot and "went down within seconds of each other," but only four of them were able to "crawl to safety," Spurlock said.
"When [Parrish] was shot and the other officers went down around him and they tried to pull him out but were unable to," he said. "[He] continued to shoot at the officers over Zack."
A Code Red was sent out regarding this incident. Any citizens in the affected area are instructed to shelter in place, avoid windows and stay away from exterior walls. pic.twitter.com/RgutCEqgJA— DC Sheriff (@dcsheriff) December 31, 2017
Spurlock said the suspect "had encounters with [the] law throughout [the Denver] Metro area."
The officers were all wearing bulletproof vests, Spurlock said, but somehow the "rounds struck them outside where the vests cover."
He said that attempts were made to evacuate residents to safety, despite the fact that the suspect shot two civilians who lived in neighboring apartment units.
"I know officers were working to get residents on either side of the [suspect's] apartment evacuated," Spurlock said.
Investigators are questioning the other tenant who let the cops into the apartment. As of Sunday, Spurlock said he was being cooperative.
They are also expected to review bodycam footage and question the officers about the deadly incident, he added.
As a tribute to Parrish Sunday, a hearse bearing the American flag left Littleton Hospital trailed by a convoy of law enforcement SUVs and cruisers with their lights on.
The scene was filmed and posted on the Douglas County sheriff's Facebook page under the heading: "Motorcade for our fallen deputy."