But four men are still on the loose and police are combing the Oxfordshire countryside surrounding the centre.
Inspector Graham Sutherland, of Thames Valley Police, said that police and GPO knew how the men had escaped but could not release details.
"What I would say is that this wasn't a simple walk out. It was more planned and complicated than that. This was a planned escape".
The men escaped through the back of the centre, which is situated on the outskirts of Kidlington and close to Oxford. Police threw a cordon around the centre and 22 officers are searching the area, including outbuildings and sheds.
Details and pictures of the men have been circulated to officers throughout the Thames Valley Police area.
GOOD LUCK TO THE ESCAPEES!