When the state police force the convicts back into the prison by launching a barrage of tear gas at them, the police move forward, into the area being bombarded. The convicts are overcome by the gas, but the police aren't - even though they're not wearing gas masks and are enshrouded by the same gas the convicts are.
At the beginning of the film, a guard is walking down the cell block stairs while looking down. As is clear in several shots, those stairs are open-metal mesh, so he would have clearly seen the group of convicts moving underneath him, under the stairs.