Experts from Norway’s Accident Investigations Board are reviewing how a cruise ship carrying more than 1,000 passengers ended up in the North Atlantic during a significant storm.

Last weekend, the Viking Ocean Cruise’s Viking Sky suffered a serious propulsion malfunction during exceptionally rough seas. The ship’s captain issued a mayday call on Saturday, as he feared the vessel might have capsized or been blown into the shallows.

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A massive rescue operation began shortly after the mayday call. The Norwegian Coast Guard dispatched helicopters to hoist passengers from the deck of the ship, which had anchored off of Hustadvika Bay in Norway about half a mile from shore, according to a report from the Associated Press (AP). During the rescue, the ship was bombarded by large swells, and passengers inside reported the vessel took moderate damage in the storm, with many windows blown out.

“When the windows and door flew open and two meters [six feet] of water swept people and tables 20–to–30 feet, that was the breaker. I said to myself, ‘This is it,’” said Rodney Horgen, an American citizen traveling aboard the Viking Sky, while speaking to AP reporters. “I grabbed my wife but I couldn’t hold on. And she was thrown across the room. And then she got thrown back again by the wave coming back.”

Other passengers posted pictures and videos of the incident on social media.

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The rescue mission took the majority of the weekend. In total, about half the passengers were evacuated by helicopter. Despite the rough seas and ship damage, the master of the Viking Sky was able to limp the vessel into port at Molde by late Sunday evening. In total, 20 people suffered injuries from the ordeal.

“It was a nightmare,” one crew member from the Viking Sky told USA Today, who wished to remain anonymous. “People were panicking, everybody was running, some people were crying. People are shocked, you can see it in their faces. They can’t believe they survived.”

As the dust settles around the disaster, investigators are looking to uncover not only why the ship’s master ignored weather warnings, but also what exactly caused the ship’s propulsion issues. Although legally, ships’ masters are responsible for the safety and wellbeing of all souls on board, as of this writing, Norwegian police hasn’t filed criminal charges.