FORT HOOD, Texas — For the seasoned doctors and nurses at Carl R. Darnall Army Medical Center, caring for wounded soldiers is something they know all too well. They did it every day during combat deployments in Iraq and Afghanistan.

But few were prepared for the grim and chaotic scene that began unfolding early Thursday afternoon as ambulance after ambulance began ferrying gunshot victims to the hospital emergency room. These were soldiers being gunned down here in the United States, amid the presumed security of one of the largest military installations in the world.

"In Iraq, you're prepared for these things to happen," said Maj. Stephen Beckwith, who spent 15 months in Iraq and now serves as director of emergency services at Darnall. "It's obviously frustrating and tragic to see these things happen on post."

Beckwith and other "first-responders" from the Darnell medical team on Friday recounted those harrowing moments after an army psychiatrist reportedly distraught about an impending overseas assignment opened fire in a processing center crowded with soldiers. Twelve soldiers and one civilian were killed. Thirty people were wounded.

As they do countless times in dealing with battlefield casualties, the Darnell medics put their emotions on hold and let their training take over as they scrambled to treat the wounded, they recalled Friday. Several used the term “controlled chaos” to describe the scene inside the hospital.

“We had no idea how many people to expect,” said Capt. Reis Ritz, an ER physician. “We had no idea who was shooting them. It was very surreal to think that so many people could be injured. We did wonder when is this going to stop.”

Ritz said he asked a woman patient, “Who shot you?”

“ ‘I don’t know,’” he quoted her as saying. “Somebody just started shooting, and they don’t know who it was or where it was coming from.’ ”

Kim Dembrosky, a 26-year-old registered nurse whose husband is deployed in Iraq, recalled a jumble of thoughts as the patients poured in. “How many more or coming?” she asked herself. “Are we safe here? Is their pain under control?”

“We were so focused on our jobs, it was pretty controlled emotions,” she said. But now, she added, “everything is starting to hit.”

Lt. Col. Larry Masullo, acting chief at Darnall, said the medical team dealt with more than 20 patients within 30 minutes. Many, he said, had multiple gunshot wound. Up to 50 doctors and nurses were hurled into the effort to treat the wounded, Masullo said.

There were “several critical points that, without the heroic efforts of our doctors, they might not have made it,” he said.

Sgt. Andrew Hagerman, a military policeman, rushed to the scene of the shooting. “You had gunshot victims everywhere,” he said. Bystanders, he said, were breaking tables to use for stretchers and ripping off their uniforms to use as bandages. He also saw the assailant, identified as Malik Nadal Hasan, lying on the ground unconscious after he had been shot.

“You don’t expect it here. You always have the sense that nothing’s going to happen here at home,” he said. “Whenever you hear about a soldier being killed, it bothers you. It really doesn’t matter who does the shooting.”