Four people have been killed inside a house in Aurora, Colorado, including the gunman. It remains unclear whether officers shot the suspect or if he shot himself.
"We're just getting in there with our crime scene detectives, so obviously we'll have to determine if it was our rounds or his rounds,'' Police Sgt. Cassidee Carlson told AP. "This is a big investigation, and a lot is entailed.''
Investigators say the three victims appeared to have been killed before police arrived at the scene.
A fifth person escaped uninjured before officers arrived and reported that she saw three bodies which "appeared lifeless," Carlson said.
A SWAT team was called after gunshots were heard at the home around 3am local time.
The gunman barricaded himself inside the house for six hours, and was killed around 9am.
SWAT teams deployed what may have been tear gas inside the home around 8:30am, in an effort to get the suspect to come out.
Officers initially tried to negotiate with the man using a bullhorn.
Several homes in the area were evacuated by police teams for safety reasons.
The names and ages of the victims have not yet been released.
Aurora was the site of a mass shooting July 20 inside a movie theater during a midnight screening of the film 'The Dark Knight Rises.' The shooting left 12 people dead.