Interpol issued an alert on Wednesday for a French inmate who was freed during a violent ambush of a prison convoy a day earlier, an attack that left two guards dead, deeply shocked France and set off a large-scale police manhunt.
It was not immediately clear whether the alert, known as a red notice, meant that French investigators believed that the inmate — Mohamed Amra, 30, born in the northern city of Rouen and nicknamed “The Fly” — had fled abroad or was trying to.
But as the hours ticked by and no sign of the suspects publicly emerged, the French authorities avowed that they were going to great lengths to find them.
“We’re putting considerable resources into it,” Gérald Darmanin, France’s interior minister, told RTL radio the morning after the ambush, which a small group of assailants staged at a tollbooth on a major highway about 85 miles northwest of Paris.
Over 450 officers, he said, had searched the area of the country where the assailants used two cars to block the prison convoy before emerging with automatic weapons and firing repeatedly, killing two guards and injuring three others before fleeing with the freed inmate.
Mr. Darmanin said it was unclear how many assailants had taken part in the ambush, although security camera footage and bystander videos that were spread on social media after the attack suggested that there were at least five. He did not say whether investigators had identified them.
Mr. Darmanin also said that the investigation involved “international cooperation,” although he did not elaborate.
Interpol, an international organization that helps police agencies worldwide share information about fugitives and crimes, said it had issued the red notice at France’s request.
The notice — a request made by law enforcement in one country, asking their foreign counterparts to locate and arrest a suspect — says that Mr. Amra is about 5-foot-8 to 5-foot-11, with brown hair and dark brown eyes. Grainy pictures accompanying the notice show him standing in a gray and white tracksuit.
Mr. Darmanin said the riskiness of the attack and the amount of preparation that appeared to have gone into its planning were surprising, given that Mr. Amra was not high-profile, despite a lengthy criminal record.
“The violence, the massacre, the disproportionate means used to free this person,” Mr. Darmanin said, did not match what the authorities knew of Mr. Amra, whom the interior minister described as “not the biggest criminal that we have in our prisons.”
Mr. Amra was not in a maximum-security prison, and the prison authorities had not requested a police escort during his roughly one-hour transfer on Tuesday between a courthouse in Rouen and a prison in Évreux.
He has been convicted 13 times for offenses including extortion and assault, as well as several thefts, according to the top Paris prosecutor. His most recent conviction was for burglary, and he has not been convicted on any drug-related charges.
But Mr. Darmanin said that Mr. Amra was also under investigation in Marseille, in southern France, in connection with a drug-related kidnapping and homicide case, noting that traffickers sometimes outsourced hit jobs to people who were not directly part of the drug trade.
As the ambush and ensuing manhunt drew international attention, French prison guards’ unions expressed dismay and outrage over the attack, which they said reflected dangerous working conditions fueled by an unsustainable level of violence in overcrowded prisons.
As of last month, there were nearly 77,500 inmates in France, but room for fewer than 62,000, according to official statistics. And France’s official prison watchdog has in recent years described a worsening “climate of violence” in the country’s prisons.
Erwan Saoudi, a representative of the Force Ouvrière Justice union, told France 2 television on Wednesday that at the prison where he works in Paris, inmate occupancy stood at 170 percent of capacity, with staffing at just 80 percent of normal levels.
“The balance of power is off,” Mr. Saoudi said, adding, “We’ve been sounding the alarm for years about this increased level of violence.”
Transporting inmates outside prisons used to mostly be a police responsibility in France, but was gradually shifted to prison guards in the past decade and is still relatively new for them, Mr. Saoudi said. Prison guards say their equipment — handguns and vans — does little to deter assailants with powerful cars and heavy weapons.
On Wednesday morning, hundreds of guards symbolically blocked prisons around France and observed a moment of silence. They demanded increased security during inmate transfers and a reduction in outside transportation of prisoners — for instance, by having legal officials come into prisons for certain procedures instead, or by using videoconferencing when possible.
