The incident occurred early Tuesday morning, Weld County, Colo., Sheriff's Office spokesman Tim Schwartz said at a press conference.
Schwartz said a 911 dispatcher who was telephoned from the home could hear gunshots in the background of the call.
Police found two males and two females dead inside the house, the Denver Post reported. Schwartz said one of the female victims may be a teenager.
"We're just trying to piece it all together at this point," said Schwartz.
The home is in a subdivision named Longview Estates, KMGH-TV, Denver, said.
Neighbors Joyce and George Vibbert said the house was occupied by a young couple who moved in three months earlier.