Police in Paris have linked a 7.65mm gun to four separate murders in the Essone area of the capital since November, raising the possibility of another serial killer following the death of Mohamed Merah in Toulouse on 22 March.

The fourth victim, a 47-year-old woman of Algerian origin, was shot four times in the head on Thursday. The gunman was seen to flee on a motorbike.

Interior Minister Claude Guéant told radio Europe-1: "This series of killings deserves our maximum attention and we're putting all our resources into this affair." Prosecutor Marie-Suzanne Le Quéau told a press conference that police were trying to determine whether there the victims were linked, and whether there was one killer or more than one: "On the theory of a serial killer, I will simply say that three of the murders – the second, third and fourth, show similarities."

However, it was also stressed that as yet no terrorism link had been made, unlike in the seven murders around Toulouse by Merah, who also used a motorbike.

Le Quéau said that a suspect in the first attack was still being held, but had retracted a confession. The fourth victim, Nadjia Lahsene, was shot while in the entrance hall of her housing block in the district of Grigny. A neighbour was quoted as saying: "Everyone is in shock. She didn't feel threatened. She's a normal person, simple, no history." Police appealed for witnesses who saw the gunman, described as tall and slim.

The first victim, Nathalie Davids, a 35-year-old lab assistant, was shot in her block's carpark in Grigny on 27 November. On 22 February, her 52-year old neighbour, Jean-Yves Bonnerue, was killed in the entrance to their building. The third, an 81-year old man, was shot in the suburb of Ris-Orangis on 19 March. 19th.

Le Queau said that over 100 officers have been deployed to investigate the case and carry out identity checks in the area of the attacks. All were killed execution-style, with shots in the head. Le Quéau also said that all four deaths occurred at the same time of day, around 4 to 6 pm. Their locations are also near two trunk roads, allowing a fast escape.

Following the first murder, a man aged 46 was arrested in December; he had been jilted during an affair with the woman, and had a record for petty crime. While in custody and with his lawyer, the man had confessed, but he then made a retraction in front of the investigating judge.

The killings come as France is still distressed by the terror attacks in the south that left dead three Jewish children and a rabbi, plus three paratroopers. Mohamed Merah, the al Qaeda-inspired gunman, also used a powerful motorbike. He was identified, put under siege in his flat, and shot dead while leaping out of the window.

France is holding the first round of its presidential election on 22 April, and the Toulouse case has played into a sharpening of the tone in the campaign; an opinion poll put Socialist François Hollande's lead over President Nicolas Sarkozy at its narrowest so far.