A 68-year-old man has been arrested by French police on suspicion of murdering 18 people, most of them homosexuals, between 1980 and 2002, officials say.
French police sealed off the entrance to Nicolas Panard's home
The suspect, Nicolas Panard, is alleged to have killed 11 people in the Alsace region, four others in neighbouring Franche-Comte, and three near Paris.
Mr Panard was detained on Tuesday in the eastern Alsace city of Mulhouse.
His alleged accomplice, Slim Fezzani, is currently serving a 20-year prison sentence for a separate murder.
Several other people are also wanted in connection with the case.
'No formal proof'
Police said Mr Panard and Mr Fezzani were currently being questioned by police in the Franche-Comte town of Montbeliard.
The deputy state prosecutor of Montbeliard, Jean-Marc Gervason, told the AFP news agency that the two men had denied the accusations and that there was "no formal proof" linking them to the crimes.
Mr Gervason said more information would become available on Thursday afternoon, when he is due to hold a news conference.
According to the local L'Alsace newspaper, the arrests were the result of two years' work by a police officer based in Montbeliard who had originally only been investigating a murder in 1991 in nearby Sochaux.
By using a police crime database, the officer found mentions of Mr Panard's name in the case files of several unsolved murders, the paper reported.
The officer also discovered several common features in the murders, including that the victims had all been killed by blows to the head followed by multiple knife wounds, it added. Each time the victims' bodies were found partly naked, but with their faces covered.
Judges had in the past dismissed several of the murder cases due to a lack of evidence.
Mr Panard, who already has a criminal record for burglary, is said to have long frequented the gay scene and worked as a female impersonator in the eastern France and Germany, the paper reported.
During the 1980s, there was a spate of unsolved murders of homosexuals across the border from Mulhouse in the French-speaking part of Switzerland.