The threat of building collapse has delayed the investigation into what caused Sunday's explosion and fire at a San Marcos business.

The fire ripped through Holland Motor Homes on Travelers Way seriously damaging the business along with at least seven motor homes parked nearby.

Neighbors gathered to see the smoke and flames after the explosion rattled the area around 4 p.m.

“This is the biggest commercial structure fire that I've seen since I've been here,” said witness Dave Clark.

At 4:45 p.m., crews upgraded their response to a third alarm fire. The fire was so big crews did not fight the fire from within. Firefighters stayed at a safe distance, trying to contain the fire to just one building.

California Highway Patrol officers issued a SigAlert due to heavy smoke causing gridlock along State Route 78.

Hazmat came in to make sure no one got hurt.

"It is a RV repair facility and RV sales facility, so all of the RV's have propane tanks we don't know what exactly exploded but that's what we are guessing right now," said San Marcos Fire Chief Brett Van Wey.

He said safety of firefighters and keeping the fire contained from a nearby storage facility were priorities.

The business was closed when the fire broke out and there were no reported injuries.

The owner was on the scene after the fire to survey the damage.

“It's painful to see this because it means a business is going to be going out, it means there will be a loss of business and it's just going to hurt the community overall,” said Ryan Deslauriers.

As of Monday, San Marcos Fire officials were not able to conduct an investigation because the structure of the parts and maintenance building was unstable.