Police say a man who fatally shot six people at a Colorado birthday party before killing himself was upset after not being invited to the weekend gathering thrown by his girlfriend’s family.

Colorado Springs police said the shooter, 28-year-old Teodoro Macias, had been in a relationship with one of the victims for about a year and had a history of controlling and jealous behavior.

The force added there were no reported incidents of domestic violence during the relationship and that the shooter did not have a criminal history.

“At the core of this horrific act is domestic violence,” police chief Vince Niski said, adding that the gunman had “displayed power and control issues” in the relationship.

About a week before the shooting, there was another family gathering where there “was some sort of conflict” between the family and Macias, Mr Niski said.

Investigators do not know yet how Macias got the weapon, which police described as a Smith & Wesson handgun originally purchased by someone else in 2014 at a local gun store and not reported stolen.

Macias had two 15-round magazines, one of which was empty, and police recovered 17 spent shells at the scene.

The shooting occurred at a home in the Canterbury Mobile Home Park on the east side of Colorado Springs, the state’s second-largest city.

Police have identified Macias’s victims as members of his girlfriend Sandra Ibarra’s extended family. Three children at the party, ages 2, 5 and 11, were not hurt.