Grenoble prosecutor Jean-Yves Coquillat said an examining magistrate would be appointed to conduct the investigation in the coming days.
Coquillat has said the accident was almost certainly the result of brake failure and the magistrate will be charged with establishing where responsibility for that lies.
The coach crashed on Tuesday on a bend near the end of a steep descent from the ski resort of Alpe-d'Huez. It was carrying dozens of young Britons who had just finished working the winter season in various ski stations.
The bus which belonged to British company Classic Coaches was carrying young Britons aged mostly in their 20s, who had just finished working during the skiing season. It was travelling down a road from the popular resort of Alpe d'Huez when it hit a cliff face at turn 21at around 1.45pm on Tuesday. It is not clear what caused the accident but investigators believed brake failure was the probable cause.
"It is very probably an issue with the brakes. Now the question is why and the investigation is likely to take a long time," Grenoble prosecutor
Jean-Yves Coquillat told AFP.
According to passenger accounts, the British driver had shouted out that the brakes were not working as the coach hurtled towards the last of a series of 21 treacherous hairpin bends that regularly feature in the Tour de France cycling race.
The driver has been praised for his "remarkable courage" which "saved many lives." Four of the coach's passengers - British seasonal workers - were left seriously injured.
The bus which belonged to British company Classic Coaches was carrying young Britons aged mostly in their 20s, who had just finished working during the skiing season. It was travelling down a road from the popular resort of Alpe d'Huez when it hit a cliff face at turn 21. It is not clear what caused the accident.
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"The bus was going down. The driver had time to warn passengers that he had a problem. He hit a parapet and the bus caught fire," said Jean Rampon, an official from the prefect of Isère, the department where Alpe d'Huez is located.
The driver, based in the North East of England died in the crash, four others were seriously injured and 22 more slightly hurt.