(CNN) -- Investigators in France are hunting for a suspected serial killer after four fatal shootings with the same weapon in a Paris suburb in the past five months.
The fourth victim, a 47-year-old woman, was shot in the head Thursday in Essonne, a few miles south of the capital, officials said.
Police are carrying out traffic stops in the area Saturday and police presence has increased to five times their usual level, CNN affiliate BFM-TV reported.
Officers are looking for a blue-and-white Suzuki motorbike in connection with the latest killing, BFM-TV said.
Interior Minister Claude Gueant told French radio station Europe 1 on Friday that he feared a serial killer could be responsible.
"We are putting every effort into finding out who is behind this," he said.
Prosecutor Marie-Suzanne Le Queau told reporters Friday that 100 investigators were working on the case.
Ballistic tests indicated the same weapon was used in all four attacks, she said, according to BFM-TV.
The other three occurred on November 27, February 22 and March 17, she said.
Le Queau said the method was not identical in each killing. In the first case, the victim received several bullet wounds to the body, while in the subsequent three cases, each victim died of a shot to the head.
A suspect was arrested in November on suspicion of the first killings and remains in custody, Gueant said.
However, three more murders have been committed since with the same semiautomatic weapon.
France is still recovering from the shock of a series of gun attacks in the southwestern cities of Toulouse and Montauban last month.
Mohammed Merah, an Islamist who according to prosecutors claimed to have attended an al Qaeda training camp, shot dead seven people in three attacks. He was killed at the end of a long police siege of his apartment in Toulouse.
A series of high profile police raids on suspected Islamists across the country have followed.