One man who was arrested at Glasgow Airport was not the French refugee who was suspected eight years ago of the murder of his wife and four children.
The sensational turnaround in the case was announced on Saturday and French sources blamed the British police for the mistake.
A "partial" fingerprint test had falsely led to claims that the easyJet passenger aristocrat Xavier Dupont de Ligonnès was wanted.
There was wild speculation that De Ligonnès may have fled France after the bloodbath in 2011 and settled in Scotland with a new woman.
But "DNA testing has now proven that the man (arrested in Glasgow) is not the murder suspect," said a French investigator.
"There has been a very big mistake in Scotland and officials are conducting an internal investigation, and it was a very big mistake to state publicly that the man was Xavier Dupont de Ligonnès."
The Scottish police had previously stated that a man was in custody in connection with a European arrest warrant.
The AFP news agency said the 58-year-old allegedly traveled under a false name when the flight came from Paris to Glasgow.
The prosecutor in Nantes, France, said that fingerprints and DNA tests were performed to establish the identity of the passenger.
De Ligonnès fled in 2011, after the bodies of his wife Agnes (49) and the children Tomas (21), Arthur (18), Anne (16) and Benoit (13) were buried in the garden of the family house in Nantes. Western France, along with their two pets Labradors, Jules and Léon.
A witness told the newspaper Ouest France on Saturday that De Ligonnès had now managed to start a new life in Scotland.
"A very reliable source indicates that the refugee had remarried even in the UK," the newspaper reported.
There was no indication that this information had been forwarded to the police.
It happened after the Scottish police said the "digital fingerprint" of a man who landed on the easyJet U26884 flight in Glasgow on Friday at 14.30 matches De Ligonnès.
He used a passport stolen in 2014, owned by a man named Guillaume Joao. Originally this was claimed, but then this information was withdrawn.
Mr Joao's home in Limay, in the Yvelines department, 40 miles from Paris, was searched by the police.
"Mr. Joao does not look like De Ligonnès, and the man who was arrested in Glasgow was not," said a French investigator.
"Digital fingerprint identification was only partial," he added.
The Prosecutor of Nantes, Pierre Sennès, had demanded "caution".
Last year, police raided underground caves and abandoned potassium mines in the Roquebrune-sur-Argens area of southern France, where De Ligonnès was discovered in April 2011 by a surveillance camera.
There was nothing there to see.
Police Scotland said earlier: "On Friday, 11 October 2019, a man was arrested at Glasgow airport and remains in police custody in connection with a European arrest warrant issued by the French authorities.
"In order to confirm his identity, requests are constantly being made, and we continue to work with our colleagues in the appropriate agencies."