The preliminary conclusion was reached Saturday despite difficulties accessing the basement of the Badim hospital because of accumulated water and poor visibility. Police officials were still investigating why the generator failed.
The fire began Thursday night and spread through the hospital, forcing staff to wheel patients into the streets on stretchers. Many of the 11 dead were elderly.
Most victims died of asphyxiation as smoke filled the wards, and some died when life support equipment stopped working in the fire.
The hospital is a short walk from Maracana stadium, which was used for World Cup matches and the 2016 Rio Olympics.