Officials said Downey police confirmed there are no injuries and the building is being cleared.
Suspect in custody after reports of shots fired at Kaiser hospital in Downey
Just before noon, the LASD Norwalk Station tweeted that they are assisting Downey police with a "fluid," emergency situation and "possible active shooter" at the hospital, located in at 9333 Imperial Highway.
Sheriff's officials confirmed shortly after that a suspect was detained for questioning. Meantime, officers and deputies searched the hospital "room by room, floor by floor."
AIR7 HD was over the scene as crowds of people evacuated the building with their arms up.
Several patrol vehicles blocked streets surrounding the medical facility as the investigation continued.