Police in Breckenridge, Minn., are investigating the death of a 3-year-old boy as a homicide after he was found dead earlier this week.

The Ramsey County Medical Examiner’s Office said Friday that the boy died from blunt force trauma. However, police said it’s unclear what caused the injuries, and no arrests have been made in the child’s death.

Emergency responders were called to a report of a child not breathing at a Breckenridge apartment Tuesday. The boy, who hasn’t been identified, was pronounced dead at the scene. Officials said at the time that the cause of death was not known.

On Friday, Breckenridge police said the child’s father and his girlfriend, who haven’t been named, are cooperating with investigators, although the father was arrested on an unrelated warrant. The state Bureau of Criminal Apprehension is assisting the police department with the investigation.

Police Chief Kris Karlgaard said it’s the first child fatality in several years in Breckenridge, a city of 3,300 residents 200 miles northwest of the Twin Cities near the North Dakota border.

Earlier this week, a Fergus Falls couple were charged in nearby Otter Tail County with murder in connection with the death of a 6-year-old boy they were caring for at the request of his grandmother.

In 2016, the most recent year of state data, 26 Minnesota children died from maltreatment — the most in five years.