BUDAPEST, Hungary — A Hungarian judge Saturday ordered the arrest of a captain whose Viking river cruise ship collided with a sightseeing boat on the Danube River, sinking the tourist boat and leaving seven dead and 21 other people missing.

Authorities said water levels in Budapest are expected to fall in the coming days, aiding efforts to salvage the victims’ bodies.

The judge ordered the 64-year-old Ukrainian captain of the Viking Sigyn cruise ship formally arrested for 30 days. He said the captain could be released on bail, but prosecutors are appealing that decision.

The Viking ship collided Wednesday evening with a much smaller sightseeing boat carrying 35 people, most of them South Korean tourists, in Budapest. Seven people were rescued.

Divers have been unable to reach the tour boat wreckage due to strong currents and murky waters.

The captain is suspected of endangering water traffic causing a fatal mass disaster, which carries a sentence of two to eight years.

The captain, identified only as Yuriy C. in line with Hungarian laws, has been in custody since Thursday. His lawyers dispute that their client made any mistakes leading to the collision, which took place under a heavy rain with restricted visibility.

Defense lawyer Gabor Elo said there are no grounds to consider his client a suspect in the case, arguing that the prosecution’s request for the arrest was motivated by the fact that the captain is a Ukrainian citizen.

Elo said his client “is very sorry that he was involved in such an accident in which so many people lost their lives or are missing.”

Hungarian police spokesman Kristof Gal said the seven confirmed victims, all South Koreans, have been identified with the help of South Korean authorities by using finger and palm prints and showing photos to relatives.

Victims’ relatives and friends on Saturday visited the site of the collision under the city’s Margit Bridge. Flowers tributes and candles have been placed along the bridge.