Some alert cooks at a Temple City Denny’s restaurant helped customers get out of the burning building before it became engulfed in flames Thursday morning.

The commercial building fire was reported just before 3 a.m. at the restaurant in the 5600 block of Rosemead Boulevard, Los Angeles County Fire Department Inspector Joey Marron said.

The cooks were in the kitchen when they smelled smoke and went outside, where they saw heavy smoke coming from the roof of the building, Marron said.

They went back inside to help all of the customers and staff out of the building and called 911, Marron said.

When firefighters arrived the building was heavily involved in smoke and fire, Marron said.

Video showed the facade of the building engulfed in flames before it eventually collapsed.

“The facade came down. At that point we went into a defensive mode,” Marron said.

Investigators believe the fire started in the attic or façade area and had been burning for quite a while before the cooks noticed the smoke, Marron said.

About 40 firefighters doused the flames in just over 30 minutes, Marron said.

A firewall helped keep the flames from spreading to a bank located in the same building, Marron said.