Published: Apr 29, 2010 5:19 PM EDT

 FAIRFIELD, Calif.  - Authorities in California say an apartment fire that killed four young children was accidental.

Fairfield Assistant Fire Marshal Jerry Clark says investigators have determined the likely cause of Wednesday night's blaze in the city about 40 miles northeast of San Francisco, but they're not yet releasing that information.

The three siblings and their cousin, all between the ages of 1 and 4, were inside the apartment when the fire broke out around 9:10 p.m. Police tried to get in and rescue them but were pushed back by heavy smoke and heat. Three of the children were found dead inside. The fourth died at the hospital.

Clark says the children's mothers are sisters. Both suffered minor smoke inhalation and one also had moderate burns on her hands and forearms. The victims' names have not been released.