A 22-month-old boy who died late last summer after suffering head injuries was a victim of a homicide, authorities said Monday.

Billy Rebel Chapman died Sept. 1, after he was brought to a hospital by ambulance from his St. Cloud home in the 1300 block of University Drive SE.

A friend of the mother was watching Billy and his two older brothers at the time Billy was injured, a relative told the Star Tribune Monday.

The boy suffered complications from a blockage of blood vessels in the brain “associated with multiple blunt-force injuries,” a statement from the Hennepin County medical examiner’s office said.

There have been no arrests in the case, which Assistant Police Chief Jeff Oxton said is “actively being worked right now.”

Larry Williams, whose son Steve was married to Billy’s mother, Amy Jo Clemens, about 10 years ago before dying in 2007 from leukemia, confirmed that St. Cloud police are investigating the boy’s death as a homicide. “That’s exactly true,” he said.

Williams, who lives in Sioux Falls, S.D. and has been a minister for 35 years, said Billy’s funeral was “the hardest funeral I ever had to preach.”

“He was always a happy, energetic and spunky little boy,” Williams said. “Billy laughed and played with his brothers. He was an active little fella.”

Williams said Clemens told him she was at work at the time that Billy was injured and her sons, all age 5 and under, were being watched by a male friend. The boys’ father, Jason D. Chapman, lives in Florida and came back to Minnesota soon after Billy died, Williams said.

A call to law enforcement for medical assistance led to Billy being found in the St. Cloud home unresponsive and suffering from severe head injuries. Officers were told the child had fallen, police said.

An ambulance took the boy to St. Cloud Hospital, where investigators began examining the circumstances of his injuries and their severity. Upon determining that the boy had “substantial head trauma,” he was moved to Children’s Hospitals and Clinics of Minnesota in Minneapolis for further treatment but died.