A photo of Joana Cruz, foreground, is seen at a makeshift memorial, in Colorado Springs, on Tuesday. AP

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, police said on Tuesday, calling the shooting an act of domestic violence.

The shooter, 28-year-old Teodoro Macias, had been in a relationship with one of the victims, 28-year-old Sandra Ibarra, for about a year and had a history of controlling and jealous behaviour, Colorado Springs police Lt. Joe Frabbiele said at a news conference.

Police said there were no reported incidents of domestic violence during the relationship and that the shooter didn't have a criminal history. No protective orders were in place.

"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, Niski said.

The other victims of the shooting early on Sunday were Ibarra’s extended family. They were identified as Melvin Perez, 30; Mayra Ibarra de Perez, 33; Joana Cruz, 52; Jose Gutierrez, 21; and Jose Ibarra, 26.

Investigators don’t know yet how the shooter got the weapon, which Frabbiele described as a Smith & Wesson handgun. He said it was originally purchased by someone else in 2014 at a local gun store but was not reported stolen.

The gunman 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’s second-largest city. Three children at the party, ages 2, 5 and 11, were not hurt.

Two families were celebrating the birthdays of family members, and 10 people were inside the home when the gunman arrived "and shot all six victims in quick succession” before turning the gun on himself, Frabbiele said. The children inside were in "close proximity” to the shots fired, he said.

Police received the first of three 911 calls from inside the home. Another was made by an adult who managed to escape. Three teenagers had left the party just before the shooting, Frabbiele said.

They returned shortly after to discover what happened. Arriving officers found Jose Gutierrez gravely wounded inside; he told the officers the suspect was in the home, Frabbiele said. Gutierrez died later at a hospital. "One of the smaller children and some of the teenagers lost both parents,” Frabbiele said.

Police say the families of the victims had requested privacy. "In Colorado, we’ve had domestic terrorism incidents where lots of people were killed, we’ve had random acts like going into a King Soopers or a movie theater, but let’s not forget about the lethality of domestic violence,” Colorado Springs Mayor John Suthers said.

Associated Press