When the terrifying blasts of rapid gunfire filled an Orlando, Florida, nightclub, many clubgoers panicked or froze.
Amid the swirl of sensory overload, their response was overwhelmingly typical of people under threat.
In an emergency situation - as any first responder can attest - a victim's first challenge is overriding the paralysis brought on by extreme fear and confusion.
Imran Yousuf, a bouncer at the Pulse nightclub, had an advantage. A Marine who had served in Afghanistan, Yousuf was able to use his training to quickly identify the impending threat and remain clear-headed as people died around him, according to the Marine Corps Times. Because of the 24-year-old's decisive actions, he is being credited with saving dozens of lives.
He told CBS News that he knew something was horribly wrong as soon as he heard the familiar crack of gunfire.
It was then, he said, that his training took over.
"The initial one was three or four (shots)," Yousuf, a former sergeant who just left the Marine Corps last month, said. "That was a shock. Three of four shots go off and you could tell it was a high calibre. Everyone froze. I'm here in the back and I saw people start pouring into the back hallway, and they just sardine pack everyone."
Yousuf told CBS that he knew there was a door behind the panicked crowd, but people were too overwhelmed to unlatch it. "And I'm screaming 'Open the door! Open the door!'" Yousuf said. "And no one is moving because they are scared."
If they failed to act, the gunman could have appeared at any moment. If someone took action, they were feet from relative safety. Yousuf told CBS there was "only one choice."
"Either we all stay there and we all die, or I could take the chance of getting shot and saving everyone else, and I jumped over to open that latch a we got everyone that we can out of there."
And I'm screaming 'Open the door! Open the door!' And no one is moving because they are scared
It was a simple act of heroism, but it may have been one of the most decisive ones that took place. Asked how many people poured through the exit, Yousuf told CBS that he estimated the number was as high as 60 or 70.
"As soon as people found that door was open they kept pouring out and after that we just ran," he said.
Yousuf served in the Marine Corps from June 2010 to May 2016 as an engineer equipment electrical systems technician, according to service records obtained by the Washington Post. He deployed to Afghanistan in 2011 and most recently was assigned to 3rd Marine Logistics Group.
His military awards include the Navy and Marine Corps Achievement Medal, Afghanistan Campaign Medal, Korean Defence Service Medal and Sea Service Deployment Ribbon.
Yousuf never credited a particular type of training with steeling his nerves during the attack or helping him identify the rifle in close quarters.
During his general deployment time frame, Marines typically underwent pre-deployment training in Twentynine Palms, California., where they received what was known as Enhanced Mojave Viper training. The website MilitaryNewcomers.com describes the experience as "the most realistic, live-fire training exercise in the Marine Corps".
"The units are presented with facilities, role players and scenarios that closely replicate the environment to which they will deploy," the site adds.
During the same period, Marines training to deploy abroad were prepared for the possibility of an insider attack by Afghan forces. Such training may have borne some resemblance to the surprise attack in Orlando, though Yousuf hasn't said as much.
What he has said, according to the Times, is that he "just reacted".
"There are a lot of people naming me a hero and as a former Marine and Afghan veteran I honestly believe I reacted by instinct," he wrote on Facebook, according to the Times.
During his interview with CBS, he went a step further, wishing he could have done more during the attack.
"I wish I could have saved more to be honest," he said. "There are a lot of people that are dead . . .there are a lot of people that are dead."