FORT JACKSON, S.C. (April 9, 2015) -- Two "masked gunmen" walked into a Fort Jackson building Tuesday morning and opened fire.

The two men carried automatic weapons and walked casually through the two-story office building of the post's 81st Regional Support Command, shooting indiscriminately at Soldiers and staff. Within minutes, the floors were littered with "casualties." Some of them were dead, others seriously injured.

Although the gunpowder in the air that day was real, everything else was simulated. The day's "casualties" were Soldiers and Department of Defense civilians wearing injury prosthetics and make-up. The shooters were also Fort Jackson employees in costume, firing blanks into the air. It was part of a mass causalities exercise designed to test Fort Jackson's response to terrorist activity.

The actors for the event were mostly young Soldiers, who were issued prosthetic injuries at Darby Field before being transported by bus to the exercise location. Some of them were playing characters intended to be disabled by their injuries who needed to be carried out of the building by emergency medical teams several hours later.

The drill was for the benefit of everybody at Fort Jackson, said Dave Perkins, an antiterrorism specialist with the 81st Regional Support Command. Even though the Soldiers would spend most of the day prone on the floors of the building, he reminded them that they were making significant contributions to post security.

"We're practicing to make sure we can prevent casualties, operate as safely and efficiently as possible and save lives," he told them.

The event was also designed to challenge mutual response agreements with off-post agencies, such as the Richland County Sheriff's Office and regional medical services, said Mark Mallach, Fort Jackson installation antiterrorism officer.

Several teams entered the building to test their response skills. Post law enforcement officers were followed by the Richland County Sheriff's Office emergency response team later in the day. Once the smoke cleared from the weapons-based drills, the casualties were carried outside to a make-shift medical triage. There, emergency medical technicians tested their own response to the crisis.

Although the active shooter scenario had wrapped by lunchtime, it continued to provide learning opportunities for post security, Mallach said.

"How are we going to recover," Mallach asked, "whether it's damage to the buildings or personnel? How are we tracking patients? There are going to be five different hospitals involved with this."

The incident site would also be treated as a crime scene and involve the Army's Criminal Investigation Command and FBI.

"It has multiple moving parts," Mallach said.

Among those parts was an "intelligence build up" that took place Monday at other locations on post. Suspicious packages were left at designated locations to test how people responded. It was a way of including everyone on post in deterring terror attacks, he said.

"We're trying to raise the level of community awareness and community participation," Mallach said. "If you see something, say something. Report suspicious individuals, activities and packages."