The driver of a tram that derailed killing at least seven people is being questioned by police amid suggestions it failed to brake before taking a corner at speed.
Investigators said the vehicle was travelling at a "significantly higher speed than is permitted" and are probing whether the driver may have fallen asleep.
More than 50 people were injured when the tram left tracks and flipped on its side as it travelled through Croydon, south London, during the morning rush hour on Wednesday.
London mayor Sadiq Khan, who visited the scene on Wednesday, warned the death toll "may well increase".
British Transport Police (BTP) said forensics teams are expected to stay at the crash site until Thursday evening at the earliest and identifying the dead may be a "complex and lengthy process".
The tram was operating from New Addington to Wimbledon via Croydon when the accident happened at 6.10am.
Scenes in the wake of the crash were described as "total carnage" and "like something out of a film" after the two-carriage tram tipped over in heavy rain next to an underpass.
Survivors rescued from the wreckage said they recalled the tram failing to brake in its usual place at a bend on the track after speeding up. One said the driver told them he thought he had "blacked out".
Royal Navy veteran Kevin Snow, 57, was on his way to work in central London when the busy service overturned.
The father-of-seven from Barnsley, south Yorkshire, said the service, which he had been using regularly while working in London, failed to slow down at its usual point coming out of a tunnel between the Lloyd Park and Sandilands stops.
He told the Press Association: "Usually as you come out the tunnel you feel the brakes, but I didn't seem to at all. I thought, he should be braking in a minute.
"The next thing I knew we were on our side. Everyone was screaming and shouting, a lot of people were injured - lots couldn't move."
He said the tram slid for between eight and 10 seconds before coming to a halt.
Martin Bamford, 30, from Croydon, said he recalled the tram "speeding up", adding: "Everyone just literally went flying."
Speaking outside Croydon University Hospital, where he is being treated for fractured or broken ribs, he said people were screaming and there was "blood everywhere", describing the scene as "like something out of a film".
Asked what he had seen he added: "There was a woman that was on top of me ... I don't think she made it at all. She wasn't responsive. There was blood everywhere."
Asked about the driver, he said: "I asked him if he was okay. He said 'yeah'. I said to him 'what happened?' He said he thinks he blacked out."
The 42-year-old man, from Beckenham, was arrested on suspicion of manslaughter and taken into custody for questioning.
The tragedy is being probed by BTP and the Rail Accident Investigation Branch (RAIB).
BTP assistant chief constable Robin Smith said they were investigating whether the driver of the Wimbledon-bound tram fell asleep at the wheel, alongside "a number of factors".
Initial findings of the RAIB show that the tram came off the tracks as it was negotiating a "sharp, left-hand curve" with a speed limit of 12mph.
An RAIB spokesman said: "Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted."