Two of the four convicts who killed a prison guard and escaped from a prison in the Inner Mongolian Autonomous Region earlier this month had been planning the escape for more than a year, local police said yesterday.

Police yesterday finished interrogating Dong Jiaji, 27, and Qiao Haiqiang, 28. They escaped from No 2 Prison in Hohhot, capital city of the region, on Oct. 17. They were at large for 67 hours before being caught.

Two other inmates, Li Hongbin and Gao Bo, joined in the escape plan about half a month prior to the incident. Gao was shot dead in the police chase.

Earlier police reports had said that Li was the main planner.

The three men and Gao, who were held at the same compound, detained one of the leading inmates for labor duties, tied up a prison officer surnamed Xu and took his uniform and access card.

They then killed Xu's fellow officer, Lan Jianguo, using a paper blade they had found and hidden, police quoted the men as saying, while stressing that the confessions had not yet been proved.

Li then put on Xu's uniform and led the others, who had changed into civilian clothes, through four gates, including one that was equipped with a biometric iris identification system, police said.

The men allegedly sneaked through while a guard was passing through the gate, bypassing the security scanner.

That contradicted earlier version of the story, which, according to Wang Lijun, deputy chief of Inner Mongolia's prison bureau, was that the men used the stolen access card and a finger they sliced off from Lan to get past three gates, one of which had a fingerprint identification system.

The prison has "never had a fingerprint identification system installed in any of its gates," several guards were quoted as saying by local media.

The convicts stabbed two more police officers while completing their escape. At the prison's gate, they dragged a taxi driver out of his cab and drove away after robbing a female passenger of 700 yuan ($100). That money enabled them to hire another cab when the first one ran out of gas.

More than 12,000 armed police and security officers were dispatched for a region-wide manhunt that checked more than 47,500 people and 24,600 cars, according to local police.

The warden of the No 2 prison, which had been endorsed by the Ministry of Justice as a national model prison, was sacked after the prison break. Four other senior prison officials were also suspended.

The maximum-security prison has 447 police officers, another 268 workers and an unknown number of inmates.

Hohhot's justice bureau had earlier called the prison break a "negligent accident on the part of prison management".

The Ministry of Justice has ordered swift, thorough safety checks at all of China's prisons and work camps, stressing that officials at different levels must "take effective measures to strengthen prison management and ensure safety and stability of prisons and work camps".