BUDAPEST (Reuters) – Hungarian rescue officials said there was little chance of finding survivors after a boat with South Korean tourists on board sank on the Danube River in Budapest, with seven people confirmed dead and 21 missing.

The boat was cruising when it collided with a larger luxury passenger boat during a rainstorm on Wednesday evening, causing it to capsize and sink with 33 South Korean tourists and tour guides and two Hungarian crew aboard.

“I wouldn’t say there is no hope, rather that there is a minimal chance (of finding survivors),” Pal Gyorfi, a spokesman for the Hungarian national ambulance service, told the M1 state broadcaster.

“This is not just because of the water temperature, but (also) the strong currents in the river, the vapor above the water surface, as well as the clothes worn by the people who fell in,” he added.

Police said the smaller boat turned on its side and sank within seconds of the collision under a bridge near Hungary’s parliament building.

They said a criminal investigation was under way to determine the cause of the accident. Police declined to say if the bigger vessel, the 135-meter (443 ft) Viking Sigyn, put out any signals for help.

Police said its investigation yielded evidence that raised personal responsibility, so it questioned the Viking Sigyn captain, a 64-year-old Ukrainian, as a suspect, and later moved to take him into custody for reckless misconduct in waterborne traffic leading to mass casualties.

Police said it initiated the formal arrest of the captain, identified as C. Yuriy from Odessa.

Officials said the hull of the Mermaid, a 27-meter (89-ft) double-decker river cruise boat, had been found on the riverbed a few hundred meters from its usual mooring point.

A crane ship docked near the wreck on Thursday in preparation for recovery operations and divers prepared equipment. Police said the rescue efforts were hampered by high water levels, strong currents and bad visibility.

“Those who were trapped in the hull or were stuck underneath can be lifted only once the wreckage is pulled out,” a police statement said.

Hungarian Prime Minister Viktor Orban offered his condolences to Seoul. South Korean President Moon Jae-in said the authorities would work with the Hungarian government to investigate the cause of the accident.

“What’s most important is speed,” Moon said in Seoul.

Some South Korean relatives of those on board started to depart for Hungary. Several family members were seen at Incheon International Airport in Seoul on Thursday night.

South Korean rescue teams and officials including South Korean Foreign Minister Kang Kyung-wha also left for Budapest on Thursday. She will hold a news conference with Hungarian Foreign Minister Peter Szijjarto at 0730 GMT on Friday.

Reporting by Gergely Szakacs, Marton Dunai and Krisztina Than; Additional reporting by Hyonhee Shin, Joyce Lee, Do Gyun Kim and Minwoo Park in SEOUL; Editing by Jon Boyle, Ros Russell and Richard Chang