Firefighters are at the scene of a three-alarm fire burning in Santa Ana Thursday morning.

The fire was reported shortly after 5 a.m. in the 2700 block of Harbor Boulevard.

Crews were forced to take a defensive mode in attacking the blaze, the Orange County Fire Authority posted in a tweet.

Aerial video from Sky5 showed a large portion of the roof had collapsed as crews continued working to douse the fire.

santa ana 3-alarm fire
Fire crews battle a 3-alarm blaze in Santa Ana on Sept. 7, 2023. (KTLA)

The building housed multiple businesses including a dental office and a Waba Grill.

Crews appeared to have gained some access to the building as of 6:30 a.m.

The OCFA said that fire investigators had arrived at the scene in an updated tweet.

The cause of the fire remains under investigation.

There have been no injuries reported as a result of the incident.

Check back for updates on this developing story.