A French teacher who accompanied high school students swept away by a deadly avalanche is in police custody, a prosecutor said Thursday.

Prosecutor Jean-Yves Coquillat said an investigation has been opened for involuntary manslaughters.

According to initial reports by witnesses, the students were skiing on a ski slope that had been closed since the beginning of the season because of lack of snow. The ski slope was closed by a 50-meter long (164 feet) net with advisory in different languages that the group stepped over, Coquillat said.

Three people were killed at the Deux-Alpes ski resort Wednesday. Two were among the group of 10 French high school students who were skiing with their sports teacher and the third was believed to be a Ukrainian skier.

Earlier Thursday, a French official said that the students may have skied ahead of their teacher.

Jean-Paul Bonnetain, the top administrative official for the Isere region, urged all skiers to heed avalanche warnings. Speaking on i-Tele television Thursday, Bonnetain said "initial witness accounts describe students passing ahead of the leader."

Another regional official said the group had no guide. Gendarmes returned to the scene Thursday to investigate, according to the official, who is not authorized to be publicly named according to administration policy.