U.S. military guards discovered a 600-foot tunnel — dug with makeshift tools — leading out of the main prison facility for detainees in Iraq before anyone had the opportunity to escape, officials said Friday.

The tunnel at Camp Bucca (search) was 12 to 15 feet deep and as wide as 3 feet and had reached beyond the compound fence, said Army Maj. Flora Lee, a spokeswoman at the Army's Combined Press Information Center in Iraq said by telephone. She did not know when guards discovered the tunnel.

Camp Bucca holds 6,049 detainees, nearly two-thirds of all those in Iraq, Lee said. Situated near the southern city of Umm Qasr (search), it is one of three detainee facilities in Iraq.

A bucket cut from a water container and a shovel made of tent material were used to dig the tunnel, Lee said. The opening was under a floorboard of the compound and was concealed with dirt.

Authorities in charge of the compound realized a tunnel was under way after they found dirt in latrines and other places, Lee said. It may have been the most extensive effort aimed at a mass escape, she said.

"I'm not aware of any other instances where this has happened," Lee said. "There have been a few other attempts at digging a tunnel but nothing of this size."

U.S. guards fired on prisoners during a riot at Camp Bucca on Jan. 31, killing four detainees and injuring six others.

The guard detachment at Camp Bucca includes military police of the 105th Military Police Battalion (search) and Air Force security forces personnel with the 586th Expeditionary Security Forces Squadron, she said.