NANTERRE, France (AP) — 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.

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

Faid, a serial robber and murder convict, hasn't been seen since then.

Philippe Veroni, head of France's organized crime police unit, said Tuesday that Faid had clearly planned the prison break carefully and had a band of accomplices willing to risk everything. He said staying on the run will become increasingly expensive for Faid as he tries to avoid capture.

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