"They found nothing of any substance," an officer with the Scottish Black Watch Regiment told AFP. "On current information, our investigations have been completed."

Members of the Royal Engineers, attached to the Black Watch, moved in at first light with two heavy earthmovers as well as pneumatic drills but they abandoned the search around six hours later.

Hundreds of people swarmed into the area around the prison complex after the British left, and continued frantic efforts to make contact with anyone who might be below.

Some banged on a metal pipe leading to the ground while others dug away earth with their bare hands.

Crowds of people insisted that noises could be heard from below but it was impossible to detect any sound, according to an AFP correspondent.

SPACE.WIRE