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They thought they would be taping sprained ankles, and treating runners with heat exhaustion. Instead, they ended up doing triage in what felt like a war zone.
Dr. Pierre Rouziar was in a medical tent designated to do triage for distressed runners of the marathon.
Immediately after the blast, Rouziar and a colleague had to decide where they would be most valuable. He sent a text to his wife and kids before heading to help the injured: "There's been a bomb at the finish line, I'm going down. Pray. Pray for everyone."
Dr. Frank Brennan had served in Iraq in 2003, working as a field doctor.
"Certainly after the second explosion, I knew this was not your typical car backing into something," said Brennan.
"When we got to the scene, it was horrific," said Rouziar. "There were just bodies lying on the ground, there was blood everywhere, the thing that sticks out in your mind, the worst for me, were all the bones."
Brennan said talking out the trauma was critical to both the victims and his staff in the medical tent, who were looking to him to keep things orderly and calm as the casualties and confused bystanderrs came in looking for help.
Despite the chaos, "everyone was attended to, which was amazing," said Rouziar.
By all accounts things could have been a lot worse if people like Brennan and Rouziar had not acted as fast as they did.