A river boat cruise through the Hungarian capital of Budapest went horribly wrong on Wednesday night, when the boat hit another, bigger vessel and flipped over, sending passengers into the water.

Seven people were confirmed dead after the accident, while 21 people remain missing. Of the 35 passengers on board the sightseeing boat, 33 were South Korean nationals.

Hungarian police said the boats collided and the sightseeing boat flipped and sank in seven seconds. The captain of the largest vessel is being held by police, who are considering arresting him formally. Known only as Mr. Yuriy, the 64-year-old man from Ukraine, is being detained due to suspicious evidence found by police when they searched his vessel.

Police have continued to search for the 21 missing passengers, though the President of South Korea, Moon Jae-in, said in a statement on Wednesday that poor weather had affected the rescue efforts.

Police did manage to rescue seven passengers from the initial accident, all of whom were sent to local hospitals to be treated for injuries. Six of the passengers have now been discharged, while one passenger is still being treated for broken ribs, according to an ER doctor who spoke with CNN.

Of the seven people killed in the accident, all were of South Korean nationality. None of the passengers had been wearing life jackets, said police.