The north tower was the first to come under attack. One of the inmates in a guard’s uniform climbed to the top of the wall and crawled along beside the inner railing of the walkway so he couldn’t be seen. He verified that the description of the tower was correct. The steel entrance door, the bulletproof-glass windows, and the reinforced-concrete structure of the tower were not vulnerable. The ventilation system was.

The tear gas–and–pepper spray combination on the guards’ belts was occasionally used to get a barricaded inmate out of his cell, and the mixture had always worked. When the uniformed inmate reached the right spot beside the tower, he found the ventilation outlet where the fan pushed out stale air. It was a pipe that extended out and down, so rainwater couldn’t seep in, and had a screen so insects couldn’t creep in. He plugged the outlet by inserting a layer of paper under the screen. But there was also an intake for fresh air, and he found it by listening for the whirr of a fan on the opposite side of the tower. The intake pipe also faced downward and had a screen over its opening to keep out insects. The prisoner removed the screen.

The design included a carbon filter that looked like a can of charcoal chips with screens on both ends, to provide some protection from tear gas that might drift all the way up there during the quelling of a prison riot. The prisoner simply pulled the filter out of the pipe and set it aside. He was carrying the gas canister from his belt and the canister from the belt of his partner, and had another piece of paper folded in his pocket. He unfolded the paper and formed it into a funnel, with the larger end covering the intake. He took the two canisters out of their leather cases, pushed the first one into the small end of the funnel, and began to spray the gas into it.

For a minute or two there was nothing but the sound of the aerosol spray. Then there were noises from inside the tower. There was a voice, and then a thump, something like the sound of a chair being kicked over, and then two voices. The prisoner left the canisters and the paper and ran around to face the tower door.

The door of the tower flew open, and the two snipers burst out, coughing and rubbing their faces. They staggered a few paces from the door before they saw the uniformed inmate.

“Guys, come this way,” he called. “We’ll get you help.”

The two seemed to see only that he was wearing a guard uniform, and that he was trying to get them to follow him, so they hurried behind him as he trotted toward the stairwell.

“Did you have a tear gas grenade go off in there?”