The head-on crash of a Los Angeles commuter train and a freight train that has killed at least 18 people likely occurred because of an engineer's failure to stop at a red signal, officials said on Saturday.
"At this moment we must acknowledge that it was a Metrolink engineer that made the error that caused yesterday's accident," Denise Tyrrell, a spokeswoman for the train line, said at a news conference.
She said the engineer worked for a subcontractor used by Metrolink. It was not immediately known whether the engineer was killed or injured.
The Friday afternoon crash -- the worst commuter train crash in Los Angeles history -- also injured 135 people, including 45 who were in critical condition.
"There are 18 confirmed deaths and we are still searching. There are some bodies visible," said Los Angeles Fire Department spokesman Ronnie Villanueva.
He said workers were continuing efforts to extricate the bodies from the twisted wreckage, "but it is slow, tedious work," which could continue into Saturday night.
The force of the crash pushed the locomotive engine pulling the commuter train backward into a passenger car, and both toppled over, igniting in flames. At least seven cars from the freight train derailed, although most remained standing across the tracks.
Both trains were traveling at about 40 miles per hour (65 kph), according to Tyrrell.
Los Angeles Mayor Antonio Villaraigosa called Friday's collision "a human tragedy that is beyond words."
Once rescue efforts conclude, the National Transportation Safety Board is poised to take over investigation of the crash.
Workers were searching for the event recorders carried by the two trains and the dispatch station recording involving the crash were to be turned over to NTSB investigators on Saturday.
The event recorders -- like the so-called black boxes carried by airliners -- monitor the actions of engineers on the trains and should provide investigators with crucial information regarding the cause of the collision, Reuters reported.