(CNN) -- Of all the Boston Marathons he's worked, and he's done a half-dozen of them, Stephen Segatore figured this one would be pretty easy.
The weather was cool, so the runners probably wouldn't be at much risk for heat stroke or dehydration. Maybe he'd help people with muscle cramps or twisted ankles, but not much more than that.
Segatore, a nurse for 18 years, started his day Monday with the elite athletes in Medical Tent B toward the start of the race. Once those runners were well on their way, he transferred to Tent A at the finish line. He was talking to a group of doctors and nurses when they heard the first explosion.
Instantly, a team meant to tend to the achy and exhausted became a trauma team. The first step: those who didn't have experience with trauma stepped aside. Those with experience dashed out in the direction of the noise.
Segatore, who works in the intensive care unit at nearby Tufts Medical Center, had experience.
"I ran out and saw people who were missing legs and part of their face and part of their abdomen," he said Monday evening. "My training prepared me for what to do, but nothing can ever really prepare you for what you see."
Segatore was one of a team of dozens of doctors and nurses who volunteered at Monday's Boston Marathon. They worked quickly to stop bleeding and start IVs so patients could get into ambulances and to hospital emergency rooms. They treated dozens of patients without proper supplies for severe trauma, such as dressings and pain medications.
"No one expects to work at the Boston Marathon and end up at a terrorist attack," he said.
One of his first patients was a young woman, he thinks maybe 20 or 22 years old, whose abdomen was torn open. Her left leg was broken and facing the wrong way and she wasn't breathing. He and his colleagues did CPR on her and kept checking for a pulse, but there was none. They stopped when they realized it was futile.
She became the first patient in their makeshift morgue.
Segatore and others checked her pockets for a wallet, an ID, a cell phone, anything that would help them find her parents' names. There was nothing.
"I still don't know who she was," he said. "She had blonde hair, blue eyes, the all-American girl. She was probably a student somewhere in Boston."
"I've seen people die all over the world, but I've never been this upset or angry in my career," Segatore said. "This is the Boston Marathon. People come from all over the world and all of a sudden that world exploded on them."
Working alongside Segatore in Tent A was Jim Asaiante, a nurse in the emergency room at the UMass Memorial Medical Center in Worcester, Massachusetts. Asaiante didn't run out after the explosion. He has plenty of experience in trauma, but as a veteran of the Iraq War, he's also had plenty of experience with explosions.
"I heard the first IED (improvised explosive device), and I know there's never one. The bad guys always set up two or three," said Asaiante, an Army captain who did an 18-month tour in Iraq in 2006 and 2007.
Another victim was brought into the tent, a man with his calves and feet blown off and blood pumping out of his knees. Asaiante put a tourniquet on him, started an IV, and in 15 minutes the man, who was screaming in pain, was in an ambulance.
After the second explosion, Asaiante ran out of the tent.
"There was lots of bleeding, shrapnel, glass. It was mayhem," he said. "The injuries were very similar to Iraq."
Despite the chaos, he said the work of the doctors, nurses and EMTs was "impeccable."
"The most amazing thing was how everyone worked in tandem. They didn't even have to speak a word between each other," he said. "In 20 years of nursing, this was the most amazing two hours of nursing in my life."