(BBC) - The Valencia underground train involved in a crash on Monday that killed 41 was travelling at twice the normal speed when it derailed, officials say.
It was doing 80km/h (50 mph) at the spot where it came off the tracks, data from the train's black box show.
On Tuesday, a five-minute silence was observed across Spain in memory of the people killed in the crash.
The prime minister and the royal family are due to attend funerals which are being held in the evening.
Officials say evidence from the black box, which was recovered from the wreckage by emergency workers, suggests the train was speeding on a curved part of the track, where the speed is limited to 40 km/h (25mph).
Fernando Soto, the Secretary General of the Railway Union, told Spanish radio that the speed of the train was the main cause of the accident.
He suggested that something may have happened to the driver - perhaps a heart attack - to explain this unusual and excessive breach of the speed limit.
Valencia's authorities have discounted a tunnel collapse or a ruptured wheel as possible factors in the crash.
With the 2004 Islamist bomb attacks on Madrid's rail network still fresh in Spanish minds, the authorities ruled out any terror link to Monday's crash.
The city of Valencia is preparing to hold a state funeral for the 41 victims of the crash.
King Juan Carlos and Queen Sofia are expected in Valencia, as is Prime Minister Jose Luis Rodriguez Zapatero, who cut short a visit to India following the crash.
There were people with heads bleeding, cuts and bruises quite seriously injured
The accident comes days before Pope Benedict XVI is due to visit Valencia, with preparations being made for hundreds of thousands of pilgrims to use the city's public transport network.
Bereaved families filed through the morgue on Tuesday to identify the bodies, many of them clearly in a state of shock.
Only one of the bodies reportedly remained unidentified.
The dead include the mother of an 11-year-old girl who is among the injured, Valencia's sub-prefect Luis Felipe Martinez was quoted as saying by AFP news agency.
A bouquet of red and white carnations and burning candles could be seen outside the Jesus station.
The four-carriage train was travelling on Valencia's Number One underground route, close to a junction with another line.
It came off the rails between Plaza de Espana and Jesus stations and smashed into the walls of a tunnel near Jesus station in the eastern Spanish city.
The train had passed a safety inspection just one week before the crash, Spain's Efe news agency reports.
Unions say the train line where the accident happened is the oldest in Valencia's metro network.
Last September, three underground trains collided in Valencia on the same line, injuring 29 people.