France's SNCF state rail company said Wednesday that no more safety risks had been detected during a nationwide probe of its network after a derailment that killed six people this month.
"The checks have not detected any situations that would threaten safety," Pierre Izard, the SNCF's general manager for infrastructure, told a press conference.
The SNCF carried out extensive checks of its rail switchpoints after the accident on July 12 at the Bretigny-sur-Orge station, about 25 kilometres (15 miles) south of Paris.
The SNCF blamed the accident -- which saw an intercity train derail as it sped through the station, crashing into the platform -- on a connecting bar that had come loose at a rail switchpoint.
Izard said nearly 5,000 switchpoints had been checked since the accident.
The SNCF said its findings had been handed over to judicial and transport safety authorities, who are carrying out their own investigations into the accident.
SNCF chief Guillaume Pepy said it remained unclear how the connecting bar had come loose at the station.
"Every possibility is being considered," he said.
The accident, which also left dozens injured, raised concerns about the state of France's rail network, with some officials complaining that regional lines had suffered as funding focused on high-speed lines.
President Francois Hollande said after the accident that France must do "much more" to maintain regional lines, vowing to make them a priority for investment.
Local prosecutor Eric Lallement said Wednesday that his office had opened a formal investigation into the accident on charges of manslaughter and involuntarily causing injury.
Three investigating magistrates were named to handle the probe, he said.
"The investigation will concentrate on the reasons for the absence, tearing or loosening of the nuts" meant to attach the connecting bar to the switchpoint, he said.
Lallement said there were no indications the driver of the train, who was praised for his fast reactions during the accident, was in any way responsible.
He said the possibility of a "malicious act" had also been excluded.
Lallement confirmed that at least two thefts had been committed against passengers of the train involved in the accident.
He said thieves had been caught on video cameras leaving the station with luggage stolen after the train had crashed, and that some of the items had been recovered.
Reports emerged in the aftermath of the accident of gangs of thieves taking advantage of the crash to prey on victims and rescue workers, sparking widespread outrage. Officials have denied that large-scale thievery took place.