(CNN) -- Firefighters were battling a gas line or gas-fueled blaze Thursday evening that engulfed dozens of homes and sent bright flames and huge plumes of smoke shooting hundreds of feet into the air in a neighborhood in San Bruno, California, emergency management officials said.
"The flames are being fueled by something," said Kelly Huston of California Emergency Management. "There was a loud explosion being reported by witness and just dozens of homes on fire"
He said there were some injured and walking wounded after the fire, which started around 7 p.m. (10 p.m. ET).
"Crews are just trying to assess how many injuries they have there and what they're going to do to contain the flames," Huston said.
Investigators were still trying to determine what caused the fire.
"It was a gas station explosion, or a pipeline close to a gas station," said Jay Alan, a spokesman for California Emergency Management, citing informationg from the San Bruno Police Department.
Authorities are worried about winds that could spread the fire and embers that are falling on adjacent houses, he said. Helicopters were dropping retardant on homes to try to stop the fire's spread.
They are still assessing the area and trying to determine how many people are injured, he said.
"It's still very much an unfolding incident as we speak," he said.
CNN affiliate KGO reported that people as far as a half-mile away said they felt the boom and heat of the explosion.
An evacuation center has been set up and the Red Cross is heading to the scene to provide aid to residents, KGO said.
Church of Highlands Staff Pastor Leigh Bishop told CNN he witnessed a "devastating explosion" that caused the fast-moving fire.
Bishop said two people from his church had lost their homes and at least one person had burn injuries.
Kaiser Permanente South San Francisco hospital received three people in ambulances and four walk-ins with injuries from the fire, Administrative Services Director Nida Winnett said. Six of those people were in the hospital's intensive care unit, and three of them are suffering from burns, Winnett said.
Seton Medical Center spokeswoman Beth Nikels said three people with injuries from the fire came to the hospital, and one is being transported to a burn center.
California's state fire department is sending 25 fire engines to the area to help battle the blaze at the request of the local fire department, spokesman Daniel Berlant said.
The fire is in a populated area about two miles west of San Francisco International Airport, Huston said.
All flights and operations there were normal Thursday evening, airport spokesman Michael Brown said.
At least 5,000 customers are without electricity, KGO reported.
"I will be closely monitoring the situation with the California Emergency Management Agency and will ensure a swift investigation begins to determine the causes," California Lt. Gov. Abel Maldonado said in a statement.
CNN's Ninette Sosa, Shawn Nottingham, Anderson Cooper, Karan Olson, Sonya Hamasaki and Deanna Proeller contributed to this report.