A fast-moving fire ripped through part of a commercial building on Austin Street and 10th Street early Thursday morning.
San Antonio firefighters arrived to heavy flames and smoke just after midnight. The fire was so intense, smoke clouded over a stretch of Highway 281 near downtown.
San Antonio fire officials said the two-story building is attached to another building, but the most significant damage was done to the two-story side.
No one was inside the building at the time and damage estimates were unknown early Thursday morning.
The arson unit is investigating a cause and said the commercial building was vacant.