The regional train was carrying 186 passengers from Rouen to Caen when it derailed Friday evening in Cleon.
Authorities suspect sabotage caused a train derailment in northwestern France that injured 44 people after a piece of rail was found on the track.
The regional train was carrying 186 passengers from Rouen to Caen when it derailed Friday evening in Cleon. One carriage tipped onto its side while four others left the tracks.
An 18-year-old woman was among the injured and was evacuated by helicopter to Rouen University Hospital, police said.
A spokeswoman for the French national police communications service, known as SICOP, told The Associated Press the piece of rail found on the track was "probably" linked to the derailment. She spoke on condition of anonymity because she wasn't authorized to be publicly named in line with police policy.
"The reasons for its presence are unknown and will have to be determined by the investigation," Rouen prosecutor Sebastien Gallois said, according to Agence France-Presse.
Investigators were working to establish whether the circumstances were accidental or involved sabotage, the service said. The prosecutor's office remained cautious about drawing conclusions, according to police.
Cleon Mayor Melanie Delacour said the victims were "in a state of shock."
"The windows of the carriages that remained upright shattered, and because of the train's speed, stones flew into the carriages," she told AFP.
France's previous rail disasters include the July 2013 derailment at Brétigny-sur-Orge, south of Paris, which killed seven people.
In November 2015, a high-speed TGV train carrying out tests derailed at Eckwersheim, near Strasbourg, killing 11.
More recently, a crash involving two high-speed trains at Adamuz in southern Spain killed 46 people and injured more than 100 in January, according to the Andalusian regional government.