A French rail company has confirmed children were aboard a high-speed train that derailed during a test run in northeast France.
Authorities on Sunday confirmed the death toll had risen to 11, with 5 people still missing.
"A few children ... were among the injured," an SNCF spokesman said, without elaborating.
"The investigation should determine the number of people present on the train (and those who) were not authorised to be on it," he said.
The unprecedented crash Saturday near Strasbourg of one of France's flagship high-speed trains was "a huge shock", SNCF chief Guillaume Pepy told a news conference.
"So far the accident is inexplicable," he added.
Pepy told French radio later that the probe would determine who rode along with the test team and "in what circumstances were they allowed to board this train. SNCF does not approve this practice ... a train test is a train test."
The train ended up partially submerged in a canal in the town of Eckwersheim. On Sunday, its silver and black rear locomotive still lay in the canal under a bridge, with the next carriage straddling the bank and the water.
The 11 dead were among 49 technicians and railwaymen tasked with testing the next-generation TGV ("train a grande vitesse" or high-speed train), which was due to go into service next spring.
Twelve people remain in critical condition among the 37 injured, according to Strasbourg deputy prosecutor Alexandre Chevrier. Five people were still reported missing, he said.
Chevrier said sabotage or an attack had not been ruled out, but were considered unlikely causes.
A senior official in the Alsace region on Saturday blamed "excessive speed" for the disaster.
The worst train accident in France in recent years occurred in July 2013 when a commuter train derailed in a Paris suburb killing seven people and injuring dozens more.
Saturday's accident happened with France on high alert following a string of deadly attacks in Paris late Friday.
However there were no signs that the train derailment was anything other than an accident during testing.