A French teacher is under investigation for manslaughter after he apparently led schoolchildren onto a closed ski run shortly before a deadly avalanche.
Police are waiting to take the teacher into custody. He was seriously injured in the disaster and remains in hospital.
Two pupils and a Ukrainian tourist were killed when the avalanche struck in the Les Deux Alpes on Wednesday.
Warnings were in place after heavy snow in recent days.
The two students who died were a boy of 14 and a 16-year-old girl from Saint-Exupery high school in Lyon.
The Ukrainian man who was killed was not part of the school group.
"We have to interview the hospitalised teacher as soon as we can," Grenoble prosecutor Jean-Yves Coquillat told reporters on Thursday.
"The question is: Why were they skiing on a closed piste?"
A huge rescue operation was launched after the avalanche struck the group of 10 teenagers, their teacher and the Ukrainian, 57, on the Bellecombe piste at about 15:45 local time (14:45 GMT) on Wednesday.
The piste, which has the most difficult black rating, had been closed since the beginning of the ski season because of a lack of snow, a manager at Les Deux Alpes told local media.
Following large quantities of snow, the avalanche risk level was raised to three on a scale of five.
Officials said the danger had been well signposted on the piste, with netting blocking the top. The run is said to be situated on a particularly icy north-facing side of mountain.
But the teacher and students appear to have deliberately gone past the signs and started skiing downhill, Mr Coquillat said.
A group of people skiing above the school party on the mountain may have triggered the avalanche, he added.
Earlier French Education Minister Najat Vallaud-Belkacem promised the investigation would "throw all light" on the circumstances of the tragedy.
Visiting the students' school, the minister expressed her condolences to the victims' families and urged their peers to "stay strong".
As well as the three who died, two other pupils were found injured and the teacher was unconscious after suffering multiple broken bones.
The teacher remains seriously ill in hospital but all other members of the party are now "safe and sound", according to the French interior ministry (in French).
Students and their families lit candles and laid tributes as remaining members of the ski party returned to the school in Lyon on Wednesday evening.
French President Francois Hollande offered "sincere condolences" to the victims' families and said "the solidarity of the whole nation" was with them.
Four other people - two Lithuanians, a Spaniard and a Czech - have died since the New Year in avalanches in the French Alps, AFP reports.
The regional government has urged skiers and other mountain users to take extra care and stay on marked pistes.