For two teenage sisters, a trip to a super market in Boulder, Colorado on Monday, turned into a nightmare when a shooter started firing inside the store killing 10 people, including a police officer. The sisters had decided to accompany their father, who was to receive a COVID-19 vaccine shot at King Soopers store.

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Narrating the nightmare the girls went through, their grandfather told nbcnews.com, "There was shooting, and he (the father) saw it and got the girls down. They ran and hid upstairs in a coat closet for an hour, in the coats," he said adding, "they were communicating with my daughter through his phone, texting, and then the cops came in through the roof and were protecting them."

The girls, 13 and 14, escaped through the back of the building, grandfather Steven said. He said he talked to the youngest and she was acting strong. "It's traumatizing, too, for them," he said. "I'm really concerned for their mental and emotional health. But they're great kids, and they're super solid. and they have good parents. But this is going to be a hurdle to get over," the website reported.

The shooter, who was also injured in the incident, was later detained and is being held in custody, said Michael Dougherty, district attorney for Boulder County. Among the dead was police officer Eric Talley, 51, who was the first to respond to the scene, said police chief Maris Herold, reports AFP.

Live-streamed video earlier had showed a white middle-aged man -- shirtless and seemingly covered in blood -- detained by police and led from the supermarket.

Eyewitnesses inside the supermarket said they heard multiple gunshots before fleeing through a back entrance. "I just nearly got killed for getting a soda and a bag of chips," Ryan Borowski, who was in the store when he heard at least eight gunshots, told CNN.

"It felt amazing that everybody was helping each other out and that our instincts were on the same page and we ran... I don't know why other people didn't, and I'm sorry that they froze. I wish that this just didn't happen," he added.