Hundreds of extra police were today drafted into Paris suburbs where four murders have been committed with the same gun since November, raising fears a serial killer is on the loose.
"The main aim is to check suspicious behaviour," senior local official Michel Fuzeau told AFP, saying police would carry out "vehicle checks and identity checks" and respond to any alarms raised by the local population.
He said "several hundred" officers had been mobilised.
The massive boost to the police presence in Essonne came after the latest killing on Thursday of a 47-year-old woman in a working-class housing estate in Grigny, south of the capital.
Around 100 investigators are probing the murders, carried out between November 27 and April 5, within a 10-kilometre (six-mile) radius, of two men and two women with no known links between them.
All the victims were shot with the same small-calibre semi-automatic 7.65mm weapon.
While nothing suggested a political or religious motive for the killings, investigators have not ruled out any possibility.
They believe it could be the work of a serial killer or a hired hitman, working for one or several persons. The gunman or gunmen could be acting alone or have accomplices.
Local prosecutor Marie-Suzanne Le Queau told journalists on Friday that the method of killing "is not identical in all four cases".
"In the first case, the victim was shot in the body several times while in the other three cases we have deaths caused by a single shot to the head," she said.
The first victim was a 35-year-old laboratory assistant, also shot dead in her building in Grigny, on November 27.
A disabled, unemployed man who said he was her ex-boyfriend turned himself in, was arrested and charged, but has since retracted his confession.
On February 22, one of the first victim's neighbours, a 52-year-old man, was shot dead in their building's car park.
Then, on March 19, an 81-year-old man was killed by a shot to the head in the entrance to a similar block of flats in Grigny's neighbouring suburb of Ris-Orangis.
The killer, described by investigators as a "tall European-looking man", fired three or four shots and fled on a motorbike, possibly a blue and white Suzuki.
"We're asking ourselves: who will be next. Whom will he shoot next," said Nouar Khemissa, a young mother, at a ceremony organised by Grigny townhall.
Thierry Mandon, the mayor of Ris-Orangis, meanwhile said residents were "not terrorised, but asking questions. They are alarmed. I reassured them this morning at the market."
"No link has been made between the four victims except for the fact that the second lived in the same building as the first," Le Queau said.
Last month, the southern city of Toulouse was shocked by a string of seven killings by Mohamed Merah, a 23-year-old Frenchman from an Algerian family who declared himself to be a supporter of the Al-Qaeda militant network.
Merah killed three off-duty paratroopers, three Jewish schoolchildren and a trainee rabbi before he was cornered in his apartment and killed in a shootout with police.