Brian Shannon needed to pick up a few basic items on Monday afternoon whenhe stumbled into a nightmare.

“My girlfriend and I were going to have some friends over to watch a movie,” Shannon, 27, tells PEOPLE. “I needed Mountain Dew, some chips, and we were low on dish soap. It was really basic. I’ve run the same errand a hundred times before.”

But just as Shannon was paying for his items at the checkout register of the King Soopers grocery store in Boulder, he heard the first sound of gunshots. “My first thought was that it sounded like balloons popping,” he says, “but then I heard someone scream and a guy started to yell. And the guy behind me said, “It’s a f—ing gun.”

Shannon – and dozens of other shoppers – were in the midst of amass shootingin which a gunmankilled 10 peoplein a matter of minutes.

The scene outside Monday’s mass shooting.David Zalubowski/AP/Shutterstock

Supermarket Shooting, Boulder

“Some people dropped to the ground, and other people ran for the door,” he recalls. “An old woman fell down, and a younger woman stopped to help her up and dragged her out the front door. There was a woman with her toddler, and she just scooped the kid up and booked it towards the front door.”

Scene in Boulder, Colorado, after shooting.Helen H. Richardson/MediaNews Group/The Denver Post via Getty Images

Supermarket Shooting, Boulder

Shannon, who works as a bookkeeper for a construction company, says that he froze for a split second. “I was like, ‘Oh my God, this is actually happening,'” he recalls. “I’ve thought about what I would do before, but in the moment, I hesitated. I didn’t know whether to drop to the ground or run out. The door was maybe 20 feet away from me, I didn’t know whether I could make it without getting shot in the back.”

Shannon heard gunshots near the back of the store. “The thing is that I didn’t know whether there was just one shooter. If there was one near the front door too, I could get shot. So I put my hands over my head and just ran for it.”

Shannon says that he didn’t pay attention to the first police officer arriving – later determined to be officerEric Talley, who was fatally shot. “I was focused on other people running out. There were kids and old people around. Everyone was crying.”

People rush from the scene of Monday’s shooting in Boulder.Chet Strange/Getty Images

Boulder, Colorado

Within moments, other police officers had arrived, and were bringing shoppers to safety. “They’d come out with people and would make sure they weren’t the shooter,” he says. “But they were rescuing all sorts of people. I don’t know how many people were in the store, but it was pretty busy. I had waited maybe 10 minutes at the checkout.”

In the parking lot, shoppers gathered in hordes, staring back at the store and deciding what to do next. “There was this weird feeling. Like, do we go home? Do we stay and try to help? How do we stay out of the way?'”

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Soon, officers had the onlookers move further away from the store. Cops took everyone’s names and let them go home.

“I got home and I started to shake,” says Shannon. “It wasn’t until I got there that it really hit me that I had looked death in the eye. I thought it was my last day on earth. I’ve been a mess ever since, but I’m so grateful to be alive.”

source: people.com