An early morning fire Wednesday in the Marina District led to two firefighters crashing through the floor of the engulfed building, and a third firefighter was also hospitalized.

The fire broke out around 4:15 am at a three-story home under construction on the 2500 block of Chestnut Street, according to the SF Fire Department on Xitter. The block is near the end of the street where Chestnut meets Richardson Avenue, two blocks south of the Palace of Fine Arts.

The home where the fire began appeared to be an older one where renovations were happening inside, and it was unoccupied at the time of the fire. The fire also spread to a next-door building.

The firefight was ongoing as of 5:15 am, and the department reported that two firefighters were injured. The public was asked to avoid the area of Chestnut Street between Baker and Broderick streets.

San Francisco Fire Chief Dean Crispen told reporters at the scene, "At one point during the fire a mayday was called" when the two firefighters fell through the floor from the third to the second floor, sustaining injuries.

The third injured firefighter reportedly "over-exerted himself," per KTVU.

Crispen said 145 firefighters were on the scene, and "the fire moved very rapidly" between the two buildings because both are wood-framed — and the house where it began had exposed walls due to ongoing electrical and plumbing work.

Occupants of buildings on both sides of the fire were immediately evacuated, and they were later allowed to return to their homes around 6 am. It's unclear if anyone was displaced.

The cause of the fire is not yet known.