(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.

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