Twenty-two people were killed in the rush-hour collision
Investigators studying a train collision in California are examining whether one of the drivers was distracted by his mobile phone.
The driver on a train carrying 222 passengers apparently ignored a red signal, causing it to hit a freight train, rail company Metrolink has said.
Local TV has reported that the driver sent a text message before the crash.
At least 24 people were killed and 80 hurt in the collision - the deadliest train accident in the US in 15 years.
The force of the impact last Friday drove the passenger train's engine back inside the first carriage.
Firefighters had to cut through the roof of the train, which was a double-decker, removing chunks of metal piece by piece. Sniffer dogs were also brought in to try to detect signs of life.
Two teenage train enthusiasts who befriended the driver of the passenger train have told local TV station KCBS-TV they received a text message from him just before the crash.
The US National Transport Safety Board (NTSB), which is investigating the crash, said it had issued a court order asking telecoms firm Verizon Wireless to release records of the train driver's cell phone.
He was among those killed in the crash.
NTSB board member Katy Higgins said the inquiry was focusing on the driver's reaction to the red signal.
"The question is, did he see it as red? Did he see it as something else? Did he see it at all?" the Associated Press news agency quoted her as saying.
A leading rail safety official in California has said he intends to seek emergency laws banning rail workers from using mobile phones on the job.
Though Metrolink bans locomotive engineers, as train drivers are termed in the US, from using mobile phones at work, there is currently no federal law against such activity.
Investigators have warned it is too early to conclude exactly what caused the crash.
Other causes being studied include possible equipment failure or the chance that the driver was suddenly incapacitated.
The train was travelling from Los Angeles to Moorpark, north-west of the city, when it crashed at 1632 (2332 GMT).
It collided with a Union Pacific freight train on a curving stretch of track in Chatsworth, in the San Fernando Valley area of Los Angeles county.
The front coach of the Metrolink passenger train derailed and was crushed by the engine after the collision. Two other coaches of the train remained upright.
Aerial images of the crash scene showed teams of rescuers using ladders to reach injured people inside the mangled front coach.
The Union Pacific freight train was badly damaged in the accident. Officials say two people - the driver and the conductor - were on board that train.