An 18-year-old Thornton man is in custody for the death of his girlfriend’s 3-month-old child.

Isaiah Marquez was arrested after police responded to a home in the 9100 block of Gale Boulevard just after 10:15 p.m. Monday after receiving a report the child was not breathing and unconscious.

The child was pronounced dead at the hospital, and medical staff said the baby showed signs of “non-accidental trauma.” The Adams County Coroner’s Office will determine the official cause of death.

Marquez is not the child’s father, police say. He is currently in custody in the Adams County Detention Facility.

The cause remains under investigation.