LOS ANGELES — A commuter train believed to have been carrying an estimated 350 people during the afternoon rush collided with a freight train yesterday, killing at least 12 people, injuring dozens and trapping an unknown number of others in a mangled passenger car imploded by its own engine car.

Firefighters extinguished a blaze under part of the wreckage and were working hours after the collision to free people from the destroyed commuter car, which was left toppled on its side with the train's engine car shoved back inside it. Two other cars in the Metrolink train remained upright.

The Union Pacific freight train's locomotive was also turned onto its side, with the rest of the train splayed out like an accordion behind it.

Los Angeles fire officials say at least 12 people have been killed. As many as 135 people have been reported injured.

Dr. Marc Eckstein, the fire department's medical director, also said early today at least two more people are still trapped inside a passenger car but he isn't sure whether they are alive.

Los Angeles police Lt. John Romero says the death toll may be as high as 20.

A passenger told KNBC-TV he was talking with a fellow passenger when the crash occurred.

"Within an instant I was in my friend's lap. It was so quick. It was devastating," he said. The passenger — visibly injured, but able to walk with the aid of firefighters — said he was involved in a 2005 Metrolink crash in Glendale and was talking about it with the other passenger when yesterday's crash happened.

The trains collided in the Chatsworth area of the San Fernando Valley.

Metrolink spokeswoman Denise Tyrrell said the train left Union Station in downtown Los Angeles and was headed northwest to Moorpark in Ventura County. She couldn't confirm how many people were on the train, but said that at rush hour there are usually about 350.

Union Pacific spokeswoman Zoe Richmond said a freight train usually has a crew of two.

The federal investigation into the crash will be headed by the National Transportation Safety Board, said Steven Kulm, a spokesman for the Federal Railroad Administration. The FRA itself will look into whether any federal rail safety regulations were violated, he said.