French police are holding two suspects over four similar murders committed in a southern suburb of Paris, believed to be the work of a serial killer, BBC News reported.

The first victim was killed in November and the fourth - a 48-year-old mother - was shot dead on 5 April.

Prosecutors say the same semi-automatic gun was used in all four attacks in the Essonne district, but police have not yet found it. A motorbike has been seized, similar to one described by witnesses.

One of the suspects, aged 33, was detained as he left his parents' home in the Paris suburb of Ris-Orangis.

Police have ruled out a connection with last month's killings around Toulouse, but no motive for the latest shootings has been determined. In Toulouse a self-professed Islamist militant, Mohamed Merah, was shot dead by police, following attacks in which he murdered seven people, including three children.

In the Essonne killings the gunman fled on a motorbike each time, like in the Merah case. The seized motorbike was found less than a kilometre from the latest murder scene in Essonne, Le Parisien news website reports.

Extensive searches were carried out after the first arrest on Saturday. The fourth victim was a widow and mother of Algerian origin, shot in the foyer of her apartment block in the Grande-Borne area of Grigny, which is adjacent to Ris-Orangis. The killing followed three others.

A man turned himself in for the first killing, but then retracted his confession and the other killings took place while he was in custody.