Hennigan, who joined the force in 1990, then knew he was part of an exercise. The officers in the vests were special operations officers running the drill. Police commanders refer to them as "coaches." A coach shadows each beat officer or supervisor throughout the exercise.
At the scene, Hennigan came under the watch of a coach who stayed close by as the officer played out the scenario. Hennigan went first to a "guard" at the site -- actually a police detective performing the role.
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Hennigan learned that the guard had spotted the car under the tower and that the wide-open gate was never supposed to be left unlocked or open. With that information and the earlier radio call describing a man fleeing from the tower, Hennigan strung up police tape and summoned a supervisor.
Soon, the 6th District's watch commander of the day, Lt. Edward Burnant, arrived. Now, Burnant, too, was part of the exercise.
As the drama continued, the pace picked up, and Burnant began to feel the weight of responsibility. All at once, he was trying to question the "guard," arrange for officers to visit the water authority's headquarters, bring in more police resources and make sure the appropriate streets were being closed.
A coach recommended that Burnant get an officer to trail him, taking notes on the many developments and jotting down important phone numbers. That would make it easier to recall critical facts, the coach suggested.
"You need to know how many officers you have, what officers are in charge of traffic control," explained the coach, Lt. Alfred Durham. "You need someone taking notes for you. You can't do it all."
Burnant next decided he would establish his emergency command post down the street from the tower, and all the officers and coaches walked to the spot he selected. Then Durham asked Burnant if the location was a good one.
Burnant pulled a laminated card from his pocket. Prepared by federal authorities, the card is a quick reference guide to how far away to set up command sites from various hazards and situations.
He realized that he had positioned the command post too close to the tower and car. If a bomb exploded, police and firefighters could be hurt or killed. Burnant said he would choose another site for the command post, farther away and upwind of the tower.
The continual unfolding of events underscored the importance of paying attention to details. Burnant said he would evacuate the neighborhood because of the risks. Coaches later asked where he was putting the displaced residents. The lieutenant realized that he needed to call for Metrobuses, which can be used as temporary shelters, and said he would request that some be rushed to the scene.
Burnant also decided that he needed to alert responding officers to approach from the upwind side of the tower in case a device exploded. And he made plans for a separate media command center, farther away from the action.
He called the department's bomb squad, which joined in the drill. Burnant asked the squad to do a quick sweep of the surrounding area to make sure there were no explosives nearby. Then he sent them to the water tower, accompanied by bomb-sniffing dogs that would inspect the car.
The drill continued as if the dogs had found a bomb. Bomb squad technicians William Powell and Joseph Duran were sent in. They returned later and told Burnant they had disarmed the "bomb."
During a short rehash session at the drill site, the coaches said that Hennigan, Burnant and the others had done a solid job. They said the officers had the appropriate gear and remained on top of events.
Hennigan said the drill was helpful because it got him to focus on a situation that he had never experienced before.
"This helps prepare for the real incident," he said.
Burnant agreed, saying the event was stressful and chaotic -- just what he would expect in real life.
"It reinforced what I needed to do," Burnant said.