Police say an eight-year-old suburban Fort Worth boy threw his infant twin sisters down the stairs of their home — killing one and injuring the other.

The incident happened in North Richland Hills before dawn Monday — hours after the boy's family had settled into their new home after moving from Florida.

Police say the boy has a "diminished mental capacity" but is too young to be detained. Under Texas law, a child must be at least ten years old to be in the juvenile court system.

Police say the parents discovered the injuries when they were awakened by one baby's cries.

An autopsy report shows one nine-month-old girl died from blunt force trauma of the head and brain. The Tarrant County Medical Examiner's Office ruled her death a homicide.

Her twin sister suffered head injuries and was taken to Cook Children's Medical Center in Fort Worth, but her condition isn't available.

Authorities said the boy is in the care of his family.