The fire happened at an apartment building near Olive and Eighth in Central Fresno around 1 a.m. Saturday morning.
Firefighters say they found a marijuana grow in the unit where the flames started, so police are now investigating. Two juveniles who jumped out of the second story window were taken to the hospital with minor injuries.
All of the apartment units had to be shut down after the fire leaving 40 people without a place to live.
Investigators are still working to figure out what sparked the fire.