A 1-year-old boy was beaten to death in a Brooklyn housing complex, authorities said Friday.

Kadein O’Neill was found unconscious by his mother last Saturday inside their Gravesend Houses apartment, according to police.

EMS rushed the baby to Coney Island Hospital, but he could not be saved, police said.

There were no signs of anything nefarious when police originally responded to the call, cops said.

But an autopsy from the city’s Medical Examiner’s Office Friday ruled that the boy had died of blunt-force trauma to the torso, according to a spokeswoman.

The tot’s mom was questioned, but was not taken into custody, cops said.

“I’d say everyone was questioned in this,” one police source said.

The city’s Administration for Children’s Services has since taken the lead on the case, according to police.

“Our top priority is protecting the safety and wellbeing of all children in New York City,” an ACS spokesperson told The Post. “We are investigating this case with NYPD and we have taken action to secure the safety of the other children in this home.”

It was unclear what the killer used to beat the baby to death or who else lived with the mother and her boy.

The mother did not return calls.

No arrests have been made as of Friday evening.