CNN  — 

After a river boat collision in Budapest killed at least seven people and left 21 missing, a boat captain has been detained.

Thirty-five people – including 33 South Koreans – were on board a sightseeing boat when it collided with another vessel in the Hungarian capital Wednesday night, according to South Korean’s Ministry of Foreign Affairs (MOFA). The collision occurred in the city center.

After the tourist boat was hit by the larger vessel, it flipped and sank in seven seconds, according to Hungarian Police,

The captain of the other boat has been detained and his arrest proposed, Budapest police said in a statement Thursday. The 64-year-old Ukrainian citizen, named only as Mr. Yuriy, was first questioned after police found evidence that warranted suspicion during their investigation into the accident’s cause.

Budapest police say they continue to search for the 21 people who remain missing. Of the seven who were rescued, six have now been discharged from the hospital. One person remains in hospital with several broken ribs, a doctor from the ER confirmed to CNN.

Seven South Korean tourists died in the accident, of whom two have been identified. None were were wearing life jackets, authorities said.

South Korea’s President Moon Jae-in said in a statement yesterday that rescue efforts had been hampered by poor weather.

Very Good Tour’s chief operating officer, Lee Sang-moo, said at a press conference in Seoul that the group was on the fifth day of a nine-day tour around Croatia, Slovakia, Czech Republic, Hungary, Austria and Germany.

Most of the tourists were middle aged, Lee added. The oldest was in their 70s – while the youngest was born in 2013.

“I deeply apologize to the parties affected by the accident, families of victims and the people who are suffering after hearing the news this morning,” Lee said. “I understand that the boat took every precaution.”

Lee added in a later press conference that 40 family members are scheduled to leave for Budapest by Friday.

A large rescue operation was launched following the collision, which adjacent countries including Croatia, Serbia, Romania, Bulgaria and Ukraine have been asked to assist in.

The Hungarian rescue team has widened its search range to 30 kilometers downstream, MOFA said, adding that the salvage of the boat involved is expected to take days.

‘No signs of a possible issue’

Mihaly Toth, a spokesman for boating company Panorama Deck, which operated the capsized ship, said the company could not confirm why the boat sank.

“It was just an average day and this was a regular trip. We carry out thousands of tourist boat trips every day, there were no signs that something like this could happen,” Toth said.

The tourist boat, called The Mermaid, had been operating on the Danube since 2003 and there had been “no signs of a possible issue with the boat,” he added.

Rescue workers gather at the bank of the Danube River in downtown Budapest.

Panorama Deck “ensures all the necessary technical maintenance is carried out every year,” he said.

South Korea’s President Moon Jae-in has ordered a rapid response team to be dispatched to help the rescue effort in Budapest, a presidential spokesperson in Seoul said Thursday.

Hungarian Prime Minister Viktor Orban told Moon in a phone call that 200 divers and medical professionals were sent to the scene to aid the search, the South Korean government said in a statement.

South Korea’s Foreign Minister Kang Kyung-wha will head up a central disaster and safety countermeasures headquarters, which will aim to ensure smooth dialogue with the victims, families and the tour boat collision.

Abrasive damage visible on the Viking Sigyn hotelship, following its collision with the sightseeing boat.

CNN’s Doug Jones, Sharif Paget, Nada Bashir, and Emanuella Grinberg contributed to this report.