DENVER — Colorado authorities on Monday were investigating why a gunman opened fire at a weekend birthday party, killing his girlfriend and five other adults inside a home before killing himself. 

The shooting happened just after midnight Sunday in Colorado Springs, police said.

Officers arrived at the trailer home to find the six people dead and a man with serious injuries who died later at a hospital.

The shooter was the boyfriend of a female victim at a birthday party attended by friends, family and children. He drove to the home, walked inside and opened fire before killing himself, police said. 

A recording of a Colorado Springs fire dispatch call shows engine companies arriving at the scene and requesting additional backup, including ambulances, as the number of possible victims becomes clearer. The first to arrive near the scene are told to hold back because of possible gunfire, then are given an initial all clear an agonizing 8-plus minutes after the first engine was dispatched. Sirens can be heard in the background.

"Engine 11, sounds like more shots are still being fired — keep your distance," the dispatcher says at one point. Twenty seconds later the engine crew responds: "Looking like we have multiple victims," and asks for more ambulances. The dispatcher requests another engine to "2828 Preakness Way, possible multiple gunshot victims."

More than eight minutes in, the dispatcher says that "police on scene are advising that there are four victims and that you are clear to go in." Soon thereafter Engine 11 requests another ambulance; 13 minutes in the dispatcher says: "The sergeant on the scene is saying that medical is cleared to enter for all six patients. … They're saying we have six."

Neighbor Yenifer Reyes told The Denver Post she woke to the sound of many gunshots.

"I thought it was a thunderstorm," Reyes said. "Then I started hearing sirens."

Police brought children out of the trailer and put them into at least one patrol car, she said, adding that the children were "crying hysterically."

The children, who weren't hurt in the attack, have been placed with relatives, police said.

In their most recent update on the shooting on Sunday, investigators were still working to determine a motive, police said. 

The names of the victims have not been released yet. Officials were still in the process of identifying them, Sandy Wilson of the El Paso County Coroner's Office said Monday. 

"My heart breaks for the families who have lost someone they love and for the children who have lost their parents," Colorado Springs Police Chief Vince Niski said in a statement.

It was Colorado's worst mass shooting since a gunman killed 10 people at a Boulder supermarket March 22.

"The tragic shooting in Colorado Springs is devastating," Gov. Jared Polis said Sunday, "especially as many of us are spending the day celebrating the women in our lives who have made us the people we are today."

Colorado Springs, population 465,000, is Colorado's second-biggest city after Denver. 

In 2015, a man shot three people to death at random before dying in a shootout with police in Colorado Springs on Halloween. 

Less than a month later, a man killed three people, including a police officer, and injured eight others in a shooting at a Planned Parenthood clinic in the city.