Officers arrived at a trailer to find six dead adults and a man with serious injuries who died later at a hospital, the Colorado Springs Gazette reported.
The suspected shooter was the boyfriend of a female victim at the party attended by friends, family and children.
He walked inside and opened fire before shooting himself, police said.
The birthday party was for one of the people killed, police said.
Police on Sunday hadn’t released the identities of the shooter or victims.
Authorities say a motive wasn’t immediately known.
Children at the attack weren’t hurt and were placed with relatives.
“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.
Colorado Springs, population 465,000, is Colorado’s second-biggest city after Denver.
In 2015, a man killed three people, including a police officer, and injured eight others in a shooting at a Planned Parenthood clinic in Colorado Springs.
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