Investigators are focusing on evidence suggesting a car driver committing suicide caused a rail crash that killed seven people on Saturday.
The driver was among those killed when a train hit his car at a level crossing near Ufton Nervet, Berkshire.
Police say they will examine the silver Mazda to see if there may be any other reasons why it stopped on the crossing.
Police have confirmed an off-duty policeman saw the car drive onto the crossing while the barriers were up, then saw them close with the vehicle still on the track.
He tried to alert the emergency services but was unable to stop the First Great Western train smashing into the stationary car and derailing.
Six people, including the train driver and the car driver were killed in the crash and the seventh died in hospital on Sunday.
Some newspapers have reported that the officer had a conversation with the driver of the car before it was hit by the train, pleading with him to save his life.
The Daily Express said the officer yelled at the young driver to "get out of the car now!"
But the man is thought to have replied: "No. Leave me alone. I want to die," the paper added.
The Daily Mail has reported that another witness, Mark Penston, spoke to the officer after the crash.
"The policeman said he had seen the car stop on the line. He said it was a suicide," Mr Penston told the paper.
However, Det Chief Constable Andy Trotter, of British Transport Police, said the officer's statement did not include any mention of a conversation between him and the driver of the car.
Speaking about the suicide theory, he said: "We never jump to conclusions, but obviously this is a major line of inquiry when you get such unusual behaviour.
"We won't make any assumptions. One of the reasons why we're going to examine the motor vehicle in such detail is to see if there are any other explanations for what might have happened."
Hes no better than a suicide bomber, I hope he rots in hell.