Colorado authorities are investigating why a gunman opened fire at a birthday party over the weekend, killing his girlfriend and five other adults inside a mobile home, before committing suicide.

The Colorado Springs Police Department responded to a call alerting to the shooting at 12:18 a.m. Sunday in the 2800 block of Preakness Way in Canterbury Park.

The alleged assailant was the boyfriend of a victim at the party attended by friends, family and children. The birthday party was for one of the people killed, police said.

Neighbor Yenifer Reyes told The Denver Post that she woke up to the sound of many gunshots. “I thought it was a storm,” Reyes said. “Then I started hearing sirens.”

Police pulled the children out of the trailer and into at least one patrol car, he said, adding that the children were “crying hysterically.”

The children who were at the birthday party were not injured and are now in the custody of other family members, Colorado Springs Police said in their note.

On Sunday, police had not released the identities of the shooter or the victims and said the motive was not immediately known.

“My heart breaks for the families who have lost a loved one and for the children who have lost their parents,” Colorado Springs Police Chief Vince Niski said in a statement.

This has been Colorado’s worst mass shooting since a gunman killed 10 people in a Boulder supermarket on March 22.

“The tragic shooting in Colorado Springs is devastating,” Gov. Jared Polis said Sunday, “especially since so many of us are celebrating Mother’s Day, women in our lives who have made us the people we are today.”

Colorado Springs, with a population of 465,000, is the second largest city in Colorado after Denver.

In 2015, a man shot and killed three people 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.

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