Firefighters are investigating a commercial structure fire early Wednesday in downtown Santa Rosa. 

The Santa Rosa Fire Department said the blaze, which was first reported shortly before 12:45 a.m., occurred in the 500 block of Fourth Street.

Upon arrival, firefighters found water coming from both floors of a two-story brick building that houses various businesses, the department said.

The blaze was raised to a two-alarm assignment after firefighters noticed a fire alarm was sounding inside the building and that windows of a second-floor establishment were stained with smoke, indicating there was fire inside.

Firefighters had to force their way into a stairway leading to the second floor, according to the fire department. Crews then found the fire inside the Thai House restaurant, just being checked by a sprinkler head that activated.

After firefighters extinguished the fire and cleared the area, they forced their way into Belly Left Coast Kitchen and Tap Room on the first floor, directly below where the blaze had started in the Thai House restaurant.

There was no smoke or fire from the first-floor establishment, but significant amounts of water were pouring down from the floor above it, the fire department said.

No injuries were reported by firefighters.

Only the Thai House restaurant and Belly Left Coast Kitchen and Tap Room were the building’s establishments damaged by the fire, according to the department, which added that the damage was estimated to be $150,000. Both businesses will be closed for an indefinite period.

The fire department noted the working fire sprinkler system prevented the flames from spreading throughout the building.

The investigation into the cause of the fire is ongoing, even though it looks accidental, according to firefighters.