The prosecutor said that another statement would be issued “at the end” of the detention and that the case was “subject to the secrecy of an ongoing investigation”.

Ms Bonnet added a note of caution: “We are extremely cautious about (providing) any means of identification” of the individual being questioned, she said.

“There have been a lot of arrests in this case, so we mustn’t get carried away.”

“I won’t be saying anymore until the suspect has been heard. We have already had a suicide after a police custody in this case, so must remain cautious and measured about its outcome.

“I don’t want to give anything away that identifies this person, or where he comes from.”

In June 2014, Patrice Menegaldo, a former soldier in the French Foreign Legion, took his own life in Ugine, close to Annecy, after being questioned about the case.

He left a suicide note referring to the Alps Murders, following his interrogation by the Chambery detectives.

Police later said that Mr Menegaldo had only ever been treated as a potential witness to the crime, and not a murder suspect, and had been questioned for a couple of hours.

Several other individuals have been detained for questioning but none charged.

Former local policeman Eric Devouassoux, a trained marksman who collected Second World War weapons at his home, was arrested in February 2014 in connection with the killings. He was later cleared but told French media the adverse publicity had “ruined my life”.

Meanwhile in November 2015 a motorcyclist linked spotted leaving the area at the time of the killing was identified thanks to an unusual helmet, only a few thousand of which had been made.

However, the biker said he had been on his way home after a paragliding trip, and was ruled out of the enquiry. It was described at the time as a major setback for police who had focused much of their attention on the motorcyclist.