SAN FRANCISCO (CBS SF) — A three-alarm fire caused earlier this month when third-party contractors installing cables underneath San Francisco’s Geary Boulevard struck a gas line ignited in less than ten seconds, according to a preliminary report released Wednesday by the National Transportation Safety Board.

The five Kilford Engineering Inc. employees working at the scene of the Feb. 6 incident, located along Geary Boulevard at Parker Avenue, were able to escape injury because of “audible and visual cues” that indicated that a mini-excavator had struck the line, the report said.

The contractors were installing fiber optic cables and initially used a jackhammer to remove the top layer of concrete. After the concrete was cleared, the contractors used a mini-excavator with a bucket and a hand shovel to remove the soil underneath, according to the report.

The mini-excavator then struck a part of a 2-inch gas line where it connected to a 4-inch gas line, resulting in the fire’s ignition around 1:15 p.m.

“There was just a fireball,” said Lisa Frankfort who was working inside a building just steps from the explosion.

Damaged gas line from massive SF fire (PG&E)

Police officers and firefighters responded and were able to evacuate about 100 people from the area, the report said.

PG&E crews arrived about 20 minutes later and began excavation on the 4-inch polyethylene gas main, which took about an hour.

In order to shut off the gas, PG&E crews had to turn off six street-level valves located at different nearby points and mechanically squeeze off the 4-inch main, the report said.

Many wondered that day why it took crews more than two hours to shut off the valves, including Frankfort.

“It didn’t seem like whatever the firemen were attempting to do, it wasn’t, you know, diminishing the fire,” she said. “It was frightening because the flames kept going higher and higher.”

During that time, firefighters cordoned off the area and applied water to the flames.

The final valve was closed at about 3:35 p.m. and the gas-fueled blaze was extinguished just before 3:40 p.m., according to the report.

Frankfort said she ran inside her building after seeing the flames to make sure no one else was inside. She said the aging network of pipelines underneath San Francisco are concerning.

“The chance for flame leaping from building to building, and we live in a very windy city, so I wouldn’t be surprised if it happened again,” she said.

After that fire was extinguished, firefighters then began containing fires started by the fire at nearby buildings.

No one was injured in the fire, however fire and police officials said two buildings were left with major structural damage and three others sustained extensive water damage.

Although a preliminary report has been released, the NTSB investigation is ongoing. NTSB investigators will now look into the preparedness and qualifications of Kilford to perform the excavation work and how PG&E, police and fire officials executed the emergency response.

Investigators will also be working with the California Public Utilities Commission, the Pipeline and Hazardous Materials Safety Administration, California Underground Facilities Safe Excavation Board, PG&E and the San Francisco Fire Department.

Immediately following the fire, PG&E officials said the contractors had called 811 to check for any utilities underneath the area before the digging commenced and PG&E sent crews to the area to make markings.

A lawsuit filed last week by two San Francisco residents who said they were left homeless by the blaze has named Kilford as a defendant, as well as MasTec Services Company Inc., MasTec Renewables Construction Company Inc., Advanced Fiber Works Inc. and Verizon Communications Inc.

The suit alleges the fire and subsequent explosion were a direct result of the defendants’ “reckless and willful violation of California law of using a backhoe to dig a trench near subsurface installations.”

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