The electrical and plumbing supply business was open at the time, and the fire forced about 20 workers out into the street.
It took more than 60 Oakland firefighters several hours to knock down the three-alarm blaze that left the warehouse totally gutted with the roof collapsed inward.
"We believe it started in a debris pile as an exterior fire only that then went into an open window of the warehouse," said Deputy Fire Chief Melinda Drayton with the Oakland Fire Department.
"The fire started at the backside of the building," said Jesus Perez, who is part of a crew remodeling the building next door.
Perez took photos of the fire just after it started in the area where a homeless man has been camping in the alley behind his building and the American Emperor.
"There's homeless people living on the backside for a long time, like two months," said Perez. "We begged them to leave because we knew this was going to happen anytime."
Once the fire was out, City of Oakland crews moved in to clear debris, while engineers examined the decimated American Emperor to determine if it was in danger of collapse.
"They know my family. They know my kids. I know them," said contractor Michael Mills. "They taught me to say 'Good morning, how are you doing?' I'm going to miss them. I hope they rebuild."