For Military Police Sgt. Andrew Hagerman, it began with a call over the radio: "Shots fired." And then another: "Officer down." He put on his lights and sirens and raced to the scene.

Hagerman, 27, was one of the first responders to the 30-minute barrage of gunfire that pierced through Fort Hood Thursday, turning the sun-drenched Texas military base into a war zone on U.S. soil. He and his colleagues described the scene as one of organized chaos, and they told of the heroism of a base full of soldiers who rushed to save the wounded, and learned that the shooter was one of their own.

SLIDESHOW: Deadly Fort Hood Rampage

"There were people on the ground, there were soldiers from all over the post rushing in with whatever they had to control the bleeding, ripping off their uniforms, their shirts, shorts, anything they could get their hands on," said Hagerman, who has served two tours in Iraq.

"They were also treating the shooter on scene," he said.

"I moved around to see who needed to be moved where, triaging the victims while also working to preserve the crime scene.

"You don't expect it here. We're trained for this, we know what to do, but it was a shocking moment."

The gunfire came to an end after civilian police Sgt. Kimberly Munley took the gunman down, despite being shot herself. When Hagerman arrived on the scene he saw a wounded Munley being carried into an ambulance.

"She's a nice person, she's straightforward and she does her job well," he said. "Was I surprised that she was able to stop him? Not at all."

First responders were quick to treat the shooter, identified as 39-year-old Army psychiatrist Maj. Nidal Malik Hasan.

When asked if there was any hesitation to provide medical treatment to a man who allegedly killed his comrades, Hagerman said, "They are always your brothers and sisters in arms no matter what. It doesn't really matter if he's doing the killing."

Hasan worked at Fort Hood's Darnall Army Medical Center, the same hospital where many of the victims were treated.

Emergency room physician Captain Reis Ritz was in Darnall when the loudspeaker came on: "Mass casualties. Mass casualties."

He said the first few to arrive were soldiers with multiple gunshot wounds who had driven themselves to the hospital. Others arrived soon after, some of them carrying friends who were more severely wounded. Many of the victims came into the emergency room with multiple gunshot wounds.

For Major Stephen Beckwith, the Emergency Medical Response director at Darnall, the sheer number of gunshot wounds struck him immediately, reminding him of blast injuries he'd seen in combat.

He and other ER personnel told FoxNews.com that the gunshot wounds appeared to have been inflicted by semi-automatic pistols loaded with long bullets more often used with a standard M16 rifle.

"Just so many gunshot wounds — gunshot wounds to the torso, the belly, the chest," he said. "It's similar to what you'd see down range."

Ritz has not yet been deployed overseas, and he said it was like nothing he'd ever seen.

"The worst part," he said, "there's all these multiple gunshot wounds, all the victims shot in multiple places, and they keep coming in and I have no idea who's shooting, where they're shooting from, or why.

"The worst part, not knowing when it would end, not knowing how many more, not knowing if it's only going to be gunshots or something else."

"It's not like we're in Iraq or in Afghanistan or in anything — it's home. It's like you'd expect in war, but it's home."