The Aurora Police Department (APD) said it received a report of a shooting at a house downtown at roughly 2:00 a.m. local time (0800 GMT). Officers found four adults, three men and one woman, dead from gunshot wounds when they arrived three minutes later, Xinhua reported.
The suspect was identified as Joseph Castorena, 21, supposedly the domestic partner of the woman killed in the house.
A relative of the killed woman told local MSNBC 9 news channel that it was an ambushed shooting. The suspect hid in the house and opened fires when the victims returned from a party.
Shortly after the shooting, a reverse 911 alert was issued in a one-mile radius for all residences to shelter in place, while officers searched the neighborhood in the dark.
The APD classified this investigation as a homicide case but warned the suspect was dangerous and may have a gun.