Once they heard the shots ring out on Friday night at Crocus City Hall, Efim Fidrya and his wife ran down to the building’s basement and hid with three others in a bathroom.

They listened as shots rang out and thousands of people who had come to a sold-out rock concert on Moscow’s outskirts began screaming and trying to flee.

Horrified and scared, Mr. Fidrya did the only thing he could think to do: He held on tight to the bathroom door, which didn’t lock, trying to protect the group in case the assailants came to find them.

“While we could hear shooting and screaming, I stood the whole time holding the bathroom door shut,” Mr. Fidrya, an academic, said in a phone interview from Moscow. “The others were standing in the corner so that if someone started shooting through the door, they wouldn’t be in the line of fire.”

They didn’t know it then, but they were sheltering from what became Russia’s deadliest terror attack in two decades, after four armed gunmen had entered the popular concert venue and began shooting rapid-fire weapons.

Their story is one of many harrowing accounts that have emerged in the days since the attack, which killed at least 137 people. More than 100 injured people are hospitalized, some in critical condition, health officials said.

Mr. Fidrya’s small group waited and waited, but the attackers had started a fire in the complex and it was spreading. Mr. Fidrya’s wife, Olga, showed everyone how to wet their T-shirts and hold them to their faces so they could breathe without inhaling toxic smoke.

And then a second round of shots rang out.

After about half an hour, it was so smoky that Mr. Fidrya, 42, thought even the assailants must have left. As he ventured out, he saw the body of a dead woman lying by the escalator. Later he saw the body of another woman who had been killed in the carnage, her distraught husband standing over her.

His group went down into the parking garage and eventually emerged on the street as the emergency service workers were carrying victims from the building.

The Islamic State, through its news agency, claimed responsibility for the attack. U.S. officials said the assailants were believed to be part of ISIS-K, an Islamic State affiliate in Afghanistan. On Saturday, Russia’s Federal Security Services announced that 11 people had been detained, including four who were arrested after the car they were fleeing in was intercepted by authorities 230 miles southwest of Moscow.

In interviews, survivors described how what started as a typical Friday night out devolved into a scene of panic and terror. The venue, which seated 6,200 people, had been sold out for a show by a veteran Russian band called Piknik.

Video footage from the scene shows the assailants shooting at the entrance to the concert venue, part of a sprawling, upscale complex of buildings that also includes a shopping mall and multiple exhibition halls. They then moved into the concert hall, where they sprayed gunfire as well, videos show.

The attackers also set the building on fire using a combination of explosives and flammable liquid, Russian authorities said.

Like the Fidryas, Tatyana Farafontova initially thought the sound of the shooting was part of the show.

“Five minutes before the show was supposed to start, we heard these dull claps,” she wrote on her VK social media page. Ms. Farafontova, 38, said in a direct message on Saturday that she was still in shock and was slurring her speech after the attack.

Then the claps got closer and someone shouted that there were attackers shooting. She scrambled onto the stage with the assistance of her husband.

“At the moment when we climbed onto the stage, three people entered the hall with machine guns,” she wrote in her VK account. “They shot at everything that moved. My husband from the stage saw bluish smoke filling the hall.”

Ms. Farafontova said that being on the center of the stage made her feel exposed and targeted.

“It felt as if they were poking me in the back with the muzzle of a machine gun,” she wrote, adding, “I could feel the breath of death right behind my shoulders.”

She crawled under the curtain and eventually followed the musicians, who had already started to flee, and ran as far as she could from the building.

Up on the balcony, Aleksandr Pyankov and his wife, Anna, heard the gunshots and lay on the floor for some time before joining others who jumped up and began running to the exit.

As they fled, they encountered a woman who had slumped down on an escalator and was blocking their route. She was alive but staring blankly ahead, Mr. Pyankov, a publishing executive, said. He told her to keep running, but then turned his head and saw what she was staring at.

“I started to look,” Mr. Pyankov, 51, said in a telephone interview. “And first I saw a murdered woman sitting on the sofa, and there was a young man lying next to her. I looked around and there were groups of bodies.”

It all happened in a matter of seconds, he said, and he tried to keep fleeing.

“The worst thing is that in this situation you’re not running away from the shooting, but toward it,” he said. “Because it was already clear that there would be a fire there, we know how it would burn. And you’re just running to figure out where else to run.”

Anastasiya Volkova lost both her parents in the attack. She told 5 TV, a state channel, that she had missed a call from her mother on Friday night at around the time of the assault. When she called back, there was no response, Ms. Volkova said.

“I couldn’t answer the phone. I didn’t hear the call,” Ms. Volkova told the broadcaster, adding that her mother had been “really looking forward to this concert.”

Accounts emerging about others who died in the assault also told tales of eager concertgoers who had made special efforts to get to the show.

Irina Okisheva and her husband, Pavel Okishev, traveled hundreds of miles — making their way from Kirov, northeast of Moscow. Mr. Okishev had received the tickets as an early birthday present, the newspaper Komsomolsaya Pravda reported. He did not live to celebrate his 35th birthday, which is this week. Both he and his wife died in the attack.

And Alexander Baklemyshev, 51, had long dreamed about seeing Piknik, a heritage rock band that was playing the first of two sold-out concerts accompanied by a symphony orchestra.

Mr. Baklemyshev’s son told local media that his father had traveled solo from his hometown of Satka, some 1,000 miles east of Moscow, for the concert.

His son, Maksim, told the Russian news outlet MSK1 that his father had sent him a video of the concert hall before the attack. That was the last he had heard from him.

“There was no last conversation,” his son said. “All that was left is the video, and nothing more.”

Mr. Fidrya said he felt grateful to be alive, and that four of the assailants had been captured.

“Now there is confidence that the crime will be solved and those non-humans who organized and carried it out will be punished,” he said. “This really helps a lot.”

But images of the victims remain seared in his memory, in particular that of the husband, his back burned from the fire, standing over his dead wife outside the building as medics attended to the wounded.

The man was talking to Mr. Fidrya’s wife, Olga, saying they were from the city of Tver northwest of Moscow, had been together for 12 years and had three children.

“For us it’s all over, by and large,” Mr. Fidrya wrote in a message after the phone interview. “But for that guy who stood over the body of his wife, and for their three children, the worst is yet to come. And there are so many people like him there.”

Oleg Matsnev contributed reporting.