It was not immediately clear how many attackers there were in total.
Gunmen attacked a prison van at a motorway toll in northern France on Tuesday, killing at least two prison officers and freeing a convict who had been jailed last week.
President Emmanuel Macron vowed that everything would be done to find those behind the attack, as hundreds of security forces were deployed for a manhunt to find the attackers and the inmate who were all still at large.
Two prison officers were killed in the attack and two others were receiving urgent medical care, Paris prosecutor Laure Beccuau said.
The wounded officers' lives were in danger, Justice Minister Eric Dupond-Moretti said.
The incident took place late on Tuesday morning at a road toll in Incarville in the Eure region of northern France.
The inmate was being transported between his prison in the town of Evreux and court in the regional centre of Rouen in Normandy.
A police source said several individuals, who arrived in two vehicles, rammed the police van and then fled.
One of them was wounded, the police source said.
It was not immediately clear how many attackers there were in total.
"Everything is being done to find the perpetrators of this crime," Macron wrote on X.
"We will be uncompromising," he added, describing the attack as a "shock".
French television channels broadcast footage of the attack taken by surveillance cameras at the toll, showing a vehicle colliding head on with the prison van.
In the video, the doors of the car are flung open and several gunmen dressed in black emerge. A firefight ensues and one individual appears to be guided away from the van by the gunmen.
A vehicle believed to have been used by the attackers was later found as burned out wreck at a different location.
- 'They will be punished' -
Dupond-Moretti immediately headed to a crisis cell at his justice ministry.
"These are people for whom life counts for nothing. They will be arrested, they will be judged and they will be punished according to the crime they committed," he said.
The prison officers who died, both men, were the first to be killed in the line of duty since 1992, he added.
One of them was married and had two children while the other "left a wife who is five months pregnant", he said.
Prison officer unions announced a day of minimum service on Wednesday and asked for urgent measures to improve the safety of staff.
Dupond-Moretti said he would meet union representatives Wednesday.
The prison convoy had no police escort, according to a source close to the case.
The source said guards were not systematic and only used for high-security prisoners, which was not the case for the inmate who fled.
A unit of the GIGN elite police force has been despatched to try and apprehend the suspects.
- 'Inexcusable violence' -
Prosecutor Beccuau named the inmate as Mohamed Amra, born in 1994, saying that last week he had been convicted of aggravated robbery and also charged in a case of abduction leading to death.
The case has been handed to prosecutors from France's office for the fight against organised crime, known by their acronym JUNALCO.
But a source close to the case said that Amra was suspected of involvement in drug trafficking and suspected of ordering gangland killings.
Another source said he is suspected of being at the head of a network.
His lawyer Hugues Vigier said Amra had already made an escape attempt at the weekend by sawing the bars of his cell and said he was shocked by the "inexcusable" and "insane" violence.
"This does not correspond to the impression that I had of him," the lawyer told BFMTV.
The incident came on the same day as the French Senate published a damning report warning that government measures had been unable to prevent the flourishing of the narcotics industry in France.
Committee chairman Jerome Durain said France was "not yet a narco-state" but drug trafficking nonetheless constituted "a direct threat to the national interest" and the government's anti-drugs measures were "not up to the challenge."
Law and order is a major issue in French politics ahead of next month's European elections and the prison van ambush sparked fierce reactions from politicians, especially the far right.
"It is real savagery that hits France every day," said Jordan Bardella, the top candidate for the far-right National Rally (RN) which is leading opinion polls for the elections.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)