SUN VALLEY — A major emergency structure fire in a large, vacant and boarded-up former restaurant in the Sun Valley area burned for nearly four hours before being doused Thursday morning by firefighters.

Crews responded about 8:30 p.m. Wednesday to the former site of an Acapulco Mexican Restaurant at 8431 Sunland Blvd., near the 5 Freeway, and escalated from a greater alarm fire to a major emergency fire, Brian Humphrey with the Los Angeles Fire Department said.

Firefighters were in a defensive operation for about 50 minutes, “with large hand-held and ladder-elevated high-volume hose streams,” but began to shift to an offensive mode about 9:45 p.m., Humphrey said.

A total of 111 firefighters extinguished the flames within the highly compartmentalized 5,736-square-foot building that experienced significant structural compromise due to the fire, Humphrey said, adding that an unstable exterior wall was torn down by an LAFD excavator.

A knockdown was called at 12:24 a.m.

Nearby buildings were not damaged by fire, and no injuries were reported.

Crews were expected to remain on scene throughout the early morning hours and will be joined at daylight by a department Urban Search and Rescue team to conduct a secondary search of the premises, Humphrey said.

The cause of the fire was under investigation.