How did inmates escape prison in NE China?

China Central Television released video footage (external link on Youku.com) showing three inmates breaking out of a prison in Harbin, in Northeast China's Heilongjiang province on Sept 2. Images taken from surveillance video show one inmate walking out of his cell at 4:19am, followed by two other inmates from their cells. They then killed a guard on duty and escaped.

Two of the inmates were caught, with one still at large, but the possible security loophole in prison administration was still being questioned on the Internet.

Image taken from surveillance video