Updated at 5:15 p.m. ET

The captain of a river cruise ship that collided with a tourist sightseeing boat in the Danube River in Budapest, causing it to capsize and sink, has been taken into custody by Hungarian police.

The captain, a 64-year-old Ukrainian identified only as Yuriy C., is suspected of endangering water transport leading to a deadly mass accident, according to The Associated Press, citing a police website.

Divers and rescue workers are still trying to find 21 people in the Danube River. At least seven people are dead, and seven were rescued. The craft had been carrying a South Korean tour group.

The smaller boat had 35 people aboard — 33 tourists and a Hungarian crew of two. All seven of the dead are South Korean, Budapest police say.

The tourists' boat, called the Mermaid, capsized within seconds of colliding with the long and low Viking Sigyn cruise ship. The first emergency call didn't reach police until 10 minutes later, officials said at a news conference Thursday.

The Viking Sigyn is a four-deck ship with 95 staterooms, according to Viking River Cruises. Video from the scene shows the two vessels drawing near each other as they approach the Margaret Bridge — and that the long ship's bow comes into contact with the smaller boat's rear quarter, sending it sideways.

Police in Budapest say they are investigating the incident in the context of a criminal proceeding, adding that they will rely on experts to determine whether any wrongdoing or error played a role in the catastrophe.

The collision took place on a scenic, busy stretch of the Danube near the Hungarian Parliament building and the Margaret Bridge. While some of the smaller boat's passengers were rescued, attempts to find more survivors or victims have been complicated by darkness, rain and strong currents.

Any survivors also would have to endure the river's cool water (currently around 60 degrees). The search for anyone who had been on the boat has now been extended far south of Budapest, officials said.

Some of the bodies that were recovered were pulled out of the water several miles south of the bridge where the incident occurred, according to a map released by the police.

The incident has left families in South Korea anxiously awaiting word about loved ones who were on the river trip. A 6-year-old girl who was traveling with her mother and grandparents is believed to be the youngest victim, according to South Korea's Yonhap news agency. In another case, a 31-year-old woman survived but her younger brother is missing, the agency says.

As the magnitude of the catastrophe reached South Korea, officials there mobilized to send help to Hungary.

"At an emergency meeting, President Moon Jae-in told officials to use all available diplomatic resources to expedite search and rescue efforts," NPR's Anthony Kuhn reported from Seoul. "Foreign Minister Kang Kyung-wha will head an interagency task force and travel to Hungary."

Budapest police say the sunken ship has been found and that they plan to raise it from the riverbed.

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