At 11 p.m., the officer killed the bull elk in the area of Ninth Street and Mapleton Avenue.
The elk, known as "Big Boy," had been in the neighborhood for years and was a favorite in the neighborhood.
"I felt really lucky that he would come into my yard, browse and just have his presence. He was such a magnificent animal," neighbor Judy Stone said.
The officer says he was on routine patrol when he saw the elk. The officer says the elk appeared injured because it was limping and some of its antlers were broken off.
"In the officer's judgment, the animal needed to be humanely put down," Boulder Police spokesperson Kim Kobel said in a release.
The elk was in a residential yard when the officer shot it, another aspect that upset residents.
"I come down stairs and my mom and dad told me the police are outside and they shot him, because they told them he was being aggressive," neighbor Lara Koenig said.
Kobel says it appears the officer did not inform Boulder Police Dispatch about his intentions to kill the animal. He did not notify an on-duty supervisor or file a report on the incident.
"Since there was no record about the Boulder Police Department's involvement, it created confusion about who was responsible. We apologize for the confusion and have initiated an internal personnel investigation into the matter," Kobel said.
"We're very concerned about this," Boulder Police Chief Mark Beckner said. "There are questions that we have, I'm sure everybody has. We've heard a lot of those questions from the community, and we have those same questions."
The elk was then taken to the officer's home to be processed for meat by another officer, who was off-duty at the time.
Residents say the Colorado Parks and Wildlife Department should have been notified and made the final decision on putting down the animal.
The case is under investigation.