Paleokostas, in his forties, was charged last year with kidnapping George Mylonas, a prominent industrialist, and is believed to have millions of euros in ransom money stashed away.

Nikos Dendias, Greece's justice minister, ordered a probe into the banks accounts of all guards in the section of the prison where the two convicts were held.

"The government believes that the operation could not have succeeded without participation from within," he said on Monday.

Previous escape

The pilot, who was found bound and gagged after the escape, has said he was abducted by an armed couple and forced into the operation.

The escape came just days before Paleokostas and Rizai were due to stand trial for a similar breakout in 2006, when a hijacked helicopter landed in the jail's central courtyard.

Guards failed to avert the incident, believing the aircraft's arrival was part of a visit by prison officials.

Paleokostas was serving a 25-year sentence for kidnapping and bank robbery and awaiting trial at the time of the escape.

Rizai, an Albanian in his thirties, was serving a life term for murder, and has since also been charged over two contract killings while on the run from the June 2006 escape.

Security is being tightened around Paleokostas' brother Nikos, who masterminded the 2006 escape and is currently in prison.