Denver police say two children under 10 years old and a woman have been killed and a third child wounded in an apparent shooting at a house in the northeast part of the city.
The wounded child is in critical condition at Denver Health Medical Centre.
Denver Police Major Crimes Commander Ron Saunier said the shooting appears to have been isolated and there is no indication a shooter is on the loose.
He said: "Is there a mass killer or someone running around? At this time we don't believe so."
Neighbour Debbie Martinez told KMGH-TV she did not hear any gunshots coming from the home and was alerted to the shooting only by the sound of police sirens converging on the house shortly before 7am local time.
The neighbourhood is in industrial area, surrounded by warehouses and a trucking terminal.
Ms Martinez told KMGH-TV the home was sold to a new family about six months ago.