A sheriff's deputy was killed and four others were wounded Sunday in a shooting in suburban Denver that capped a year of deadly attacks in the United States.

Two civilians were also injured.

The shooting happened after deputies were called to the scene of a domestic disturbance in Highlands Ranch, the Douglas County Sheriff's Office said on Twitter.

The suspected gunman was also shot and is believed to be dead and "no longer a threat".

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Shots were fired in the course of the investigation at the Copper Canyon Apartments, a landscaped complex 28km south of Denver, the sheriff's office said. Residents in the vicinity were advised to stay inside and avoid exterior walls and windows.

An investigator heads to the scene of the shooting. Authorities say one deputy has died and multiple others were wounded, along with two civilians. Photo / AP
An investigator heads to the scene of the shooting. Authorities say one deputy has died and multiple others were wounded, along with two civilians. Photo / AP

A hospital where three people were taken for treatment says those people suffered noncritical injuries.

Another nearby hospital said it received four patients but wouldn't release their conditions.

The Washington Post reported authorities have not named the deceased or injured officers, or the civilians.

Deputy Jason Blanchard, spokesman for the sheriff's office, said no other information will be released until a news conference later Sunday.

Steven Silknitter, 50, said he was on his way home from work when his son called and told him there was a shooter at his apartment complex.

He rushed home, but couldn't get inside the complex because police had blocked the entrance. As he was sitting outside, calling his fiance, who was sleeping inside their apartment, shots rang out.

"After she woke up, we heard about more than a dozen gunshots, a clear exchange of volley of gunshots," Silknitter, who works as an overnight truck driver while going to school part-time, told The Washington Post.

Later, he said, he saw officers carrying a person whom he could only guess was the deceased deputy.

"Not in Highland Ranch, Colorado, not in our apartment," Silknitter said in disbelief.

"Lived here for three years, quiet, nothing out of the norm. People go to work and [have] families, and then to wake up to a barrage of gunfire just a couple of buildings away from us."

Silknitter said he doesn't know anything about the suspect.

One of the deputies injured is the son of Boulder County Sheriff Joe Pelle, the Denver Post reported, citing the sheriff's Facebook page. Boulder is more than 90 miles away from Douglas County.

A reporter for ABC affiliate Denver7 News tweeted early Sunday morning that the television station's crew heard gunshots just as they arrived at the scene.

A motorcade escorted the body of the fallen officer from a hospital on Sunday.

The shooting prompted a tweet from President Donald Trump, who offered his condolences from his Mar-a-Lago resort in Palm Beach, Florida.

He tweeted "My deepest condolences to the victims of the terrible shooting in Douglas County @DCSheriff, and their families. We love our police and law enforcement - God Bless them all! #LESM"