An engineer's error likely caused a train collision that killed 25 people near Chatsworth, Calif., on Friday, a spokesperson with the Metrolink commuter rail service said Saturday.

The company believes an engineer who worked for a subcontractor that Metrolink has used for the past decade failed to stop his train at a red signal, Denise Tyrell said.

The commuter train collided head-on with a Union Pacific freight train in the Los Angeles suburb about 4:30 p.m. local time.

It took workers nearly all day Saturday to look through the wreckage for bodies. When the search ended, they had found 24 dead.

"It was like peeling an onion to find all the victims there," Los Angeles Mayor Antonio Villaraigosa said.

The death toll hit 25 when when a man injured in the collision died in USC Medical Center on Saturday, a spokeswoman said.

Nine of the 135 people who were injured are in serious or critical condition, the Associated Press reported after surveying five hospitals on Saturday night. In total, 81 people were taken to hospital.

The engineer who may have missed the signal is believed to have died in the crash, Tyrell said.

'Horrific' crashing sound

Joelle Ouellet, a Canadian living in Chatsworth, was near the scene of the accident when it happened.

Speaking to CBC News from her home, said she was riding her horse in a nearby arena when she heard a "horrific and unbelievable crashing sound" and immediately thought a train had hit a car.

"Not long after the crash, I saw the big plume of black smoke and flames shooting up," she said.

Ouellet said she saw people lying outside the train and others wandering around in a daze. Paramedics arrived about five minutes after the crash, she said.

"I could hear them chainsawing through bushes and cutting fences to get to these passengers, and they just kept coming."

Emergency crews are calling the Chatsworth crash the worst train accident in the United States in 15 years.

Until Friday, Metrolink's worst disaster was on Jan. 26, 2005, in suburban Glendale when a man parked a gasoline-soaked sport utility vehicle on railroad tracks. A Metrolink train struck the SUV and derailed, striking another Metrolink train travelling the other way, killing 11 people and injuring about 180 others.

Juan Alvarez of Compton, Calif., was convicted over the summer for causing the crash.

That was the worst U.S. rail tragedy since March 15, 1999, when an Amtrak train hit a truck and derailed near Bourbonnais, Ill., killing 11 people and injuring more than 100.

With files from the Associated Press