French investigators believe a team of four or five loyal accomplices carried out the brazen prison helicopter escape of one of France's most notorious criminals.

Key points: French investigators say Faid had clearly planned the escape

French investigators say Faid had clearly planned the escape Faid's brother Brahim, who was visiting at the time of the escape, says he had no knowledge of the escape plan

Faid's brother Brahim, who was visiting at the time of the escape, says he had no knowledge of the escape plan Justice Minister Nicole Belloubet, who has been criticised for poor security, promises report within two weeks

The armed, masked men took a helicopter pilot hostage, forced him to land in Reau Prison's yard, used power tools to break through the prison yard's doors and hustled Redoine Faid to freedom, police investigators said.

Faid, a serial robber and murder convict, hasn't been seen since escaping the prison near Paris.

French police have launched a massive manhunt for him.

Philippe Veroni, deputy director of France's organised crime police unit, said Faid had clearly planned the prison break carefully and had a band of accomplices willing to risk everything.

"This kind of person thinks about nothing except how to escape," Mr Veroni said.

"He thinks about it, he reflects on it. It's his entire life under incarceration."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 55 seconds 55 s Gangster breaks out of Paris prison in helicopter with help of armed men

Faid's brother, Brahim, was visiting him at the time of the escape. He was detained immediately afterward and freed late Monday.

"I have nothing to do with the escape," he told BFM television.

Brahim Faid's lawyer, Keren Saffar, said Redoine told his brother that someone was coming to get him and to stay where he was.

"Brahim Faid was stunned, he was shocked by the situation. And all he did was wait for the prison guards to come," she said.

Investigators are also looking into whether Faid had help from inside the prison.

"If we didn't ask, we wouldn't be doing our jobs," Mr Veroni said.

Government under fire for lack of security

Staying on the run will become increasingly expensive for Faid as he tries to avoid capture, but authorities said they have no clear sense of how much money the serial robber of banks and armoured trucks had stashed away from his previous crimes.

Faid has escaped from behind bars before — the previous time he used explosives hidden in tissues.

It is Faid's second attempted escape after breaking out of prison for six weeks in 2013. ( AP: IBO/Sipa )

Nicole Belloubet, France's justice minister, has come under increasing criticism for the flawed security around Faid.

Speaking to sceptical lawmakers, she promised a report within two weeks detailing security shortcomings and said she would "accept all the consequences".

A prison guard union member said the helicopter landed in the only area of Reau Prison that did not have anti-helicopter protections. Months ago, a drone was spotted over the prison, which authorities now think may have been used to scope out an escape route.

The 46-year-old Faid was serving 25 years for the 2010 death of a young police officer killed during a botched robbery.

In the 1990s, he led a gang involved in robbing banks and armoured vans.

Mr Veroni would not speculate on how long Faid could remain on the run.

"There are getaways that can last even longer than the sentence itself," he said.

"That doesn't keep us from working on it."

AP