French prosecutors have placed self-confessed killer Michel Fourniret under formal investigation for the killing of a second French girl.
Fourniret's trial is expected to take place in France
He was placed under formal investigation on Monday over the rape and killing of a 13-year-old girl. Charges are expected to follow.
Fourniret, 62, has confessed to eight killings between 1987 and 2001, and is suspected of others.
He has already been charged in Belgium with two murders.
He has also admitted another attempted killing, although his wife, Monique Olivier, accuses him of a ninth murder, the AFP news agency reports.
Judicial sources say Fourniret is likely to be interviewed by French officials in Dinant, Belgium, until Thursday.
Under French law, charges are usually brought only after a judicial investigation has taken place.
Fourniret is being investigated for the kidnap, attempted rape and killing of 13-year-old Natacha Danais.
He denies the charge of attempted rape, his lawyers say.
Natacha Danais went missing as she was out shopping with her mother in November 1990 in Nantes, western France. Her body was found a few days later on a local beach.
On Monday, an investigation was also initiated into the kidnapping, rape and murder of 13-year-old Mananya Thumpong. Her body was recovered in Belgium's Ardennes in March 2002, after her disappearance in France nearly a year earlier.
French officials are also expected to question Fourniret over the deaths of Isabelle Laville in 1987, Fabienne Leroy in 1988, Jeanne-Marie Desramault in 1989 and Celine Saison in 2000.
They have said they hope to charge Fourniret in all cases.
Prosecutors say Fourniret moved to Belgium after being convicted of paedophile offences in his native France, but repeatedly re-crossed the border to commit offences in his home country.
Despite being charged with two murders in Belgium, Belgian media say he is likely to be extradited to France to stand trial there.
Monique Olivier is also being held in Belgium and is expected to be investigated by the French officials over four of the murders.