The black Peugeot 5008 rammed the police van carrying a prisoner as it emerged from a tollbooth on a major highway about 85 miles northwest of Paris. Hooded men with automatic weapons leaped from the car, encircling the van and firing on it with unhurried precision for more than two minutes.

When they were through, two prison guards were dead — the first to be killed in the line of duty in 32 years — three more were wounded, and the still-handcuffed prisoner the van was transporting, Mohamed Amra, had escaped, setting off a manhunt involving several hundred officers.

“The attack this morning, which took the lives of prison guards, is a shock to us all,” President Emmanuel Macron of France said on X after the attack, which occurred around 11 a.m. on Tuesday and stunned the country with its brazenness and violence. “We will be uncompromising,” he added, promising to track down the perpetrators.

But more than 10 hours after the ambush, no trace of the assailants, who also used a white Audi that followed the van, had been found, and Mr. Amra remained at large.

Laure Beccuau, the top Paris prosecutor, said at a news conference on Tuesday that one prison guard was still in critical condition. She said investigators were combing through a crime scene that showed signs of “extreme violence.” A national unit specialized in organized crime is leading the investigation, a move that is reserved for the most serious cases.

Ms. Beccuau said that Mr. Amra, 30, had no prior drug-related convictions. But French news outlets reported that Mr. Amra was known as La Mouche, or the Fly, and had been involved in international drug trafficking and organized crime.

“It was a war operation,” Dominique Rizet, a commentator on police affairs, told the TV network BFMTV. The French authorities have not suggested that Mr. Amra has any links to terrorism.

The attack was captured on security camera footage and video filmed by bystanders that was later posted on X. At a time when France is trying hard to project an image of law and order ahead of the Olympic Games, the images of violence on the main highway from Paris to Normandy were a blow. The attack came just days after the Olympic flame arrived to much fanfare in Marseille.

Jérôme Barbier, a resident of Incarville, France, who was on his way to his beehive about 100 yards from the tollbooth, said he heard shooting, but did not see it.

“It was a big, big shooting; it lasted for five minutes,” Mr. Barbier, 58, said in a telephone interview. “Then it calmed down for one to two minutes, and then there was an explosion. And then two more gunshots.”

Mr. Barbier, who said that he had worked for the gendarmerie — the force that oversees smaller towns and rural and suburban areas in France — in the 1980s, said he could tell it was “heavy fire.”

“It wasn’t a light weapon; it was really powerful,” he said.

Ms. Beccuau said that the black Peugeot — which passed through the tollbooth several minutes before the convoy and waited for it to arrive — had been stolen. Two other cars, including a white one, were found burned in separate locations about a dozen miles from the tollbooth. Both are believed to have been used by the assailants, she said.

Prison guards are armed with handguns and equipped with bulletproof vests during transfers, and the van transporting Mr. Amra was accompanied by another prison administration car. But no armed police escort joined the convoy on the one-hour journey from a courthouse in Rouen to a prison in Évreux.

Ms. Beccuau said that Mr. Amra, who had been transferred last month to that prison, was “very well known” by the police. He has been convicted 13 times since he was 15 years old for offenses including extortion and assault, as well as several thefts.

A court in Évreux sentenced him last week to 18 months in prison for burglary. He is also under investigation in Marseille in connection with a kidnapping and homicide case and in Rouen in connection with an attempted homicide and extortion case.

Ms. Beccuau said that the prison administration had decided several weeks ago to increase the number of officers securing Mr. Amra’s transfers. She did not say why, but noted that he was supposed to undergo a disciplinary review after prison staff members noticed what appeared to be saw marks on the bars of his cell.

Hugues Vigier, Mr. Amra’s lawyer, told BFMTV that he was “completely dumbfounded” by the attack and said it did not “fit the profile” of his client.

The attack occurred on the same day that a Senate committee completed a report on rampant drug trafficking in France and recommended the creation of a French equivalent to the U.S. Drug Enforcement Administration. It said that the government has not taken the measure “of the dimensions of the threat.”

“The extent of drug trafficking gives us the feeling that there is a relationship of strong versus weak, in which the strong are the criminal organizations and the weak is the state,” Jérôme Durain, a Socialist senator and one of the two authors of the report, told Le Monde, a French daily newspaper.

Ms. Beccuau said one of the guards who was killed was a 52-year-old father of twins with nearly three decades of experience in the prison administration. The other guard who was killed was 34 years old and expecting a child with his wife.

France’s main prison guard unions called for a symbolic shutdown of the country’s jails on Wednesday to honor their dead colleagues and to protest working conditions.

“This was an attack of an unparalleled violence, in the brutality and cowardice of the killers,” Gabriel Attal, the prime minister, told the National Assembly, which observed a minute of silence on Tuesday. “We will spare no effort or means to find them. We will track them down — and they will pay.”

Aurelien Breeden and Ségolène Le Stradic contributed reporting.