Fire investigators said squatters are to blame.
The blaze happened at a building on Washington Avenue near the BART tracks.
Flames raced through the boarded up one-story complex at 9 a.m. on Sunday.
Firefighters say the building has been vacant for some time -- and it's the second fire to break out there in as many months.
MORE: Firefighter injured battling 3-alarm commercial building fire at San Leandro 'grow house'
"I do know it was pretty vacant in there. I do know that this is a place where homeless and unhoused people have been known to seek shelter in unfortunately. This is a place where we are trying to keep people out of. It's getting more dangerous at this point," said Paige Bowie, Fire Division Chief with the Alameda County Fire Department.
The chief says she's hoping the building is torn down eventually.
Crews say they did not find anyone inside.
There are no reports of injuries.
Police are still figuring out what exactly sparked the fire.
If you're on the ABC7 News app, click here to watch live