PARIS -- A French prosecutor says the teacher who accompanied high school students swept away by a deadly avalanche in the French Alps is suspected of involuntary manslaughter.

Prosecutor Jean-Yves Coquillat in the eastern city of Grenoble, near the Alps, said the teacher is being questioned by police at a local hospital where he is treated for injuries from the avalanche.

According to initial reports by witnesses, the students were skiing on a ski slope that had been closed since the beginning of the season due to lack of snow. The ski slope was closed by a 50-meter long (164 feet) and one-meter high (3.2 feet) net with advisory in different languages, and the group deliberately stepped over it to access the slope, Coquillat said.

"This is not inattention," the prosecutor said. "It is in full knowledge that the group moved into this place and this closed slope."

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.

According to the first questioning, some of the students on Wednesday morning had asked to ski on this specific slope, but another teacher leading the school trip had refused because the slope was closed.

It is unclear why the teacher who accompanied the group went on the closed slope with the students. The prosecutor said further investigations and expertise "will also seek to determine the mental state of the teacher and his ability to supervise a group."

Coquillat noted that a large amount of snow had fallen on the resort on the previous days and that many skiers, "probably several hundred," had skied on the closed slope on Wednesday.

Some witnesses quoted by the prosecutor said that the avalanche could have been triggered by another group of skiers, tourists from Hungary and Romania, who were skiing higher up on the slope.

Earlier Thursday, another French official suggested 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.