SNCF’s general director of infrastructure, Pierre Izard, said initial investigations suggest a “fishplate” or joint bar that bolted two rails together on a track became detached.
SNCF president Guillaume Pepy said the piece would be central to investigations into the crash, but warned it is too early to jump to conclusions.
The company has ordered checks on 5,000 similar pieces on the entire network.
A minute’s silence was held at train stations across France at midday local time.
The intercity train from Paris to Limoges derailed and crashed into a platform at Brétigny-sur-Orge train station just after 5pm on Friday, local time. An emergency alert was immediately activated.
Fatal derailment at Brétigny-sur-Orge: key points
• Four men and two women between 19 and 82 years old died.
• Nine people seriously injured.
• SNCF officials believe a faulty fishplate joining two tracks near a switch track became detached and may have caused the accident.
• SNCF has ordered checks on 5,000 similar fishplates across the network.
• In May and June, SNCF carried out maintenance work on a switch track at Brétigny-sur-Orge but SNCF said it was not on the same track as the one where the train derailed.
• There will be three investigations: a judicial investigation, one by the SNCF, and a third by the Transport Ministry.
Authorities say 385 passengers were on the train at the time. Four carriages of the train jumped the tracks, of which three overturned. One carriage smashed across a platform and came to rest on a parallel track; another lay halfway across the platform.
Around 300 firefighters, 20 paramedic teams and eight helicopters were deployed to treat casualties at the scene and airlift the most seriously injured to nearby hospitals.
In total, 192 people were treated by emergency services, officials said.
Rescue teams worked through the night checking the wreckage of overturned
carriages to see if any passengers remained trapped inside.
Transport Minister Frederic Cuvillier said further deaths could not be ruled out.
Witnesses described scenes of chaos, with one survivor describing having to walk over a decapitated body to escape an overturned carriage. Images on social media showed passengers being carried out of the mangled wreckage by emergency services as shocked passengers looked on from other platforms.
Guillaume Pepy, the head of France's SNCF rail service, told reporters that
SNCF, judicial authorities and France's BEA safety agency would each investigate the cause of the derailment.
Visiting the scene shortly after the accident, French President Francois Hollande said: "We should avoid unnecessary speculation. What happened will eventually be known and the proper conclusions will be drawn."
Witnesses say the train arrived at the station at high speed, however Cuvillier said the train was travelling at 137 kilometres an hour, well below the 150 kilometres an hour limit for that part of the track.
Praising the quick action of the driver, Cuvillier ruled out driver error and said the investigation would focus instead on the "rolling stock, infrastructure and the precise signalling area".
"Fortunately the locomotive driver had absolutely extraordinary reflexes by sending the alert immediately, which avoided a collision with a train that was coming the other way and just a few seconds later would have smashed into the cars that were derailing. So it's not a human problem," he told French radio on Saturday.
The derailment was France's worst rail accident since 2008, when a train collided with a school bus, killing seven school children.