Investigations show that the Colorado gunman who shot seven people dead including himself did so because he was not invited to a birthday party.

Colorado Springs Police Department said that the 28-year-old Teodoro Macias shot six people at a birthday party including his girlfriend then killed himself at a weekend birthday party because he was not invited.

Police also said that he did not have a criminal history neither he had an issue of domestic violence in the relationship with his girlfriend.

“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.

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.