Several murders involving the same weapon are raising concerns as a serial killer following four similar murders since November maybe on the loose in a Paris suburb.
The killings have been carried out reportedly with the same gun, with the latest murder of a woman taking place on Thursday.
The murders come as France is still reeling from a wave of terror-related shootings in Toulouse by Mohammed Merah.
French authorities say there is no evidence yet of a link to terrorism in the Essonne killings, but Claude Gueant, interior minister, told French radio Europe-1 that he fears a serial killer might be behind them, and that "maximum resources" would be used to find the perpetrator.
In the Essonne killings, Marie-Suzanne Le Queau, prosecutor, said all four murders appeared to have been committed at the same time of the day, between 4pm and 6pm.
She said that similarities were especially apparent in the last three cases, in which the victims were all shot in the head.
Gun cartridges were recovered in Thursday's killing, when the shooter fled on a motorbike.
The first murder, which took place in November, involved a different method, but the same gun, Le Queau said. She said a suspect in the first attack was already behind bars, but had retracted his confession to the November murder.
Authorities were still trying to establish whether just one person is behind the killings.
In addition, questions remain about whether the victims were linked to one another. The prosecutor said the victims in the first two murders "lived in the same building," but made no clear link with the other two.
French agencies are already are on alert after a string of shootings last month in southern France that police say was the work of Merah who used a motorbike, videotaped the shootings and claimed links to al-Qaeda.
He was shot dead by police last month in the city, following attacks in which he murdered seven people, including three children.