Lille prosecutor Frederic Fevre said about 100 investigators were involved in the hunt for Redoine Faid, known for brazen attacks on cash-in-transit vehicles, after he blasted his way out of a jail in the northern town of Sequedin on Saturday.
Faid, a 40-year-old who risked a heavy new sentence over the 2010 death of a policewoman, used explosives to blast through five prison doors and took hostage four prison guards, who were later released.
Investigators were Sunday trying to determine how Faid had managed to obtain explosives and a firearm inside the prison, and who may have acted as accomplices.
“A thorough investigation has begun. Obviously he had one or more accomplices. The investigators will now determine how he was able to obtain explosives and a weapon,” Fevre told AFP.
On Sunday police detained a brother who had regularly visited Faid in prison for questioning, Fevre said.
“No potential lead can be ignored. The doors must be closed one after the other,” he said, adding that the brother had been at the prison on Saturday to meet Faid but had not been able to do so.
French officials have warned that Faid is considered armed and “especially dangerous”.
France has issued a Europe-wide arrest warrant and called in Interpol for help amid fears he may have already fled across the border into Belgium, which is only a few kilometres (miles) from the prison where Faid was being held.
A spokeswoman for Belgian Federal Police, Kaatje Natens, said roadblocks had not been ordered but that train stations and airports were being closely watched.
Police have said the breakout was planned with meticulous precision and authorities insisted there was no fault on the part of prison workers.
It began at 8:30 am (0630 GMT) on Saturday while Faid was in a visitor’s room. Armed with the firearm and explosives, he took four guards hostage and in about half an hour blasted his way out of the prison, where an accomplice was waiting in a vehicle.
All of the hostages were released unharmed, one just outside the prison, another a few hundred metres (yards) away and the last two along the highway.
The getaway car used in the escape was found burnt along the highway, where Faid is believed to have switched to a second vehicle.