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By Emmanuelle Saliba, Euronews and Gabe Gutierrez

An Army sergeant who was inside a Chattanooga recruitment center when a gunman unleashed a barrage of gunfire said military training took over after the first gunshot was heard Thursday morning.

The recruitment station was the first target of alleged gunman Mohammad Youssuf Abdulazeez, 24, who fired between 25 and 30 rounds at the storefront at around 10:45 a.m. before moving on to a Naval reserve center where he killed four Marines, authorities said.

"There was the one single shot that alerted us, and about a second or so after that the first volley of fire erupted," Sgt. Robert Dodge told NBC News Thursday night. That was followed by a second volley of gunfire, he said.

The four Marines in the recruiting station went into “active shooter” drill after the first gunshot and barricaded themselves in the back of the building.

Related: Chattanooga Gunman Was Not On Terrorism Radar: Officials

"We prepared for what would come next," he said.

Dodge, who served four tours in Iraq and is the father of one son, had only been stationed at the recruiting center for 35 days before Thursday’s attack.

The gunman never entered the recruiting station, instead driving off to a Naval/Marine Corps center where he killed four Marines and critically wound a Navy sailor before being killed himself after a shootout with police. It is unclear if he was killed by law enforcement or killed himself.

The soldiers in the recruitment station are not armed, per policy. One soldier in the recruiting station was shot in the leg.

Had the gunman entered the station, Dodge said "we would have done what ever other soldier would have done, we would have taken him out to the best of our ability, or we would have died trying."

Dodge would not comment on the policy that forbids servicemembers from being armed on the property.

However, a retired Marine who used to work at that recruiting station said he is thankful everyone in the office survived — but upset that they had no better means to defend themselves.

"Recruiting duty is supposed to be a break from going to combat, a break from having to deal with gunfire and all this stuff that happens and we are not even safe here," Robert Freeman, 28, who served tours in Iraq and Afghanistan and who worked at the recruiting office for two years before retiring as a sergeant in April, told NBC News Thursday evening.

"And we have no means of protecting ourselves," he said.

Phil Helsel contributed.