BBC Correspondent Chris Morris: "Questions will inevitably be asked about whether much more could - or should - have been done to prevent a tragedy."
The train crash south of Paris which left six people dead may have been caused by a fault in the rail tracks, says the state rail company.
SNCF said a metal bar connecting two rails had become detached close to Bretigny-sur-Orge station.
Transport Minister Frederic Cuvillier has praised the driver, saying his quick actions averted a worse accident.
Those killed were four men and two women, aged between 19 and 82. Thirty people were injured, eight seriously.
A minute's silence was held across France's train network at noon on Saturday to commemorate the victims.
The train had just left Paris on Friday afternoon and was heading for Limoges when it derailed at Bretigny-sur-Orge at 17:14 (15:14 GMT) on Friday.
Transport routes were particularly busy at the time, as France began a long weekend for Bastille Day.
Continue reading the main storySix carriages derailed as the train passed through the station at 137km/h (85mph). The train's third and fourth carriages derailed first and the others followed. One mounted the station platform.
At the scene
On a tree-lined residential street in a quiet suburb just south of the capital, women in their sandals carry plastic grocery bags, bulging with thick, fresh French baguettes.
Today they have to manoeuvre around reporters, TV satellite tracks and police security barriers. France's worst rail disaster for a quarter of a century struck at the top of their street
The quiet hum of generators blurs with summer bird song. If it weren't for the emergency haulage trucks, staffed by men in orange and yellow vests, and the odd burst of a siren, it might be difficult to believe that such a devastating event happened here just yesterday.
Blue cranes keep heaving chunks of crushed and curled metal off the tracks. Police watch, arms crossed. Hanging baskets shudder in the breeze.
Giving its initial findings, SNCF management told reporters the connector had worked its way loose and become detached at points 200m outside Bretigny station.
"It moved into the centre of the switch and in this position it prevented the normal passage of the train's wheels and it may have caused the derailment," Pierre Izard, SNCF's general manager for infrastructure, told reporters.
'Extraordinary reflexes'The inquiry is now expected to focus on how the piece of metal had become detached.
Checks are being carried out on some 5,000 similar connections across the whole of the rail network.
A crane has arrived on site to lift a carriage which was left on its side.
Regional government head Michel Fuzeau said there was a possibility that more bodies could be found underneath, but that there was "no hope of finding anyone wounded".
Aside from SNCF, investigations are being conducted by judicial authorities and France's BEA safety agency.
Mr Cuvillier said the driver had "absolutely extraordinary reflexes in that he sounded the alarm immediately, preventing a collision with another train coming in the opposite direction and which would have hit the derailing carriages within seconds".
SNCF said 385 passengers were on board when the train crashed and the station platforms were crowded.
French media comments
Francois Sergent in Liberation calls the accident a "a national tragedy" that "touches the hearts of all French people. All of us have been, at one time or another, passengers on the French rail network."
Bertille Bayart in Le Figaro says the accident, which comes days after the government announced investments in the railways, will spark controversy over infrastructure that is "characterised by 'serious degradation', in the words of the transport minister".
Olivier Razemon in Le Monde contends that the crash "must not eclipse the fact that railways remain one of the least dangerous means of transport in the world, in terms of deaths per kilometres or hour travelled".
Transport expert Alain Bonnafous on Atlantico website says it is impossible at this stage to know what caused the accident, but vandalism is always a possibility. "But few like to talk about it because it is so easy to disrupt the network," he adds.
British student Marvin Khareem Wone was on a train on another platform when the carriages of the intercity ploughed into the station.
"The train went off the railway; it just went on the platform and kind of flew in the air for a second and went upside down," he told the BBC.
"The first and the second coach were completely destroyed. I really thought no-one could survive that because it was completely mashed up. Everyone was crying and running everywhere. A woman was crying for her daughter who was still on the train."
Because of the damage to the station, he said ambulances could not reach the platform and the lift was not working.
Many people feel it was lucky that the accident was not a lot worse, given the violence of the impact and the fact that a packed train ploughed onto the platform at peak time, says the BBC's Hugh Schofield in Paris.
Local media said a group of people had attempted to steal from the victims and rescuers shortly after the crash and threw stones at emergency workers as they tried to reach passengers.
However later Mr Cuvillier said there had only been "isolated acts", including an attempt to steal a mobile phone - although small groups had given the rescuers a "somewhat rough welcome".
Journalist Olivier Ravanello was one of the first at the scene of the crash
The Red Cross and France's SAMU rescue service denied they faced any problems in their operations, the AFP news agency reports, while local socialist MP Jerome Guedj tweeted that it was necessary "not to play things down (...) but not to exaggerate anything either".
Were you at the station or on the train? Please get in touch using the form below.