BUDAPEST, Hungary — Hungarian police detained the captain of a cruise ship that collided with and sank a sightseeing boat packed with South Korean tourists, as rescue crews geared up to raise the vessel from the bottom of the Danube River in the heart of Budapest while scouring the waters for 21 people still missing.

Seven people are confirmed dead and seven have been rescued, all of them South Koreans, Hungarian officials said. Hungary’s state TV reported that all those rescued have been released from the hospital except one who is being treated for broken ribs.

Officials said preparations to bring up the 70-year-old boat, which was built in the former Soviet Union, could take days.

Police, who launched a criminal investigation into the incident, said late Thursday that they had detained and questioned the Ukrainian captain of the larger vessel.

The 64-year-old man is suspected of endangering water transport leading to a deadly mass accident. In line with Hungarian laws, the suspect was identified only as Yuriy C., referencing his first name and the initial letter of his last name. Police proposed the arrest of the Odessa resident, described earlier by police as an experienced sailor.

So far, only seven of the 35 people onboard, that included 33 South Koreans, are known to have survived the incident near the Hungarian Parliament in Budapest. Seven others are confirmed dead and 21 remain missing.

A South Korean group on a package tour of Europe — including 30 tourists, two guides and a photographer— were on an hour-long sightseeing tour of Budapest when their boat collided with a Viking cruise ship during a downpour Wednesday evening.

Nineteen South Koreans and two Hungarian crew members — the captain and his assistant — remain missing.

A map released by Hungarian police showed the locations and times the seven bodies were recovered, with one body found nearly 12 kilometers (7.5 miles) downstream, nearly 2-½ hours after the collision.

"Those contributing to the search will continue to do everything possible ... along the full Hungarian stretch of the Danube’s coasts in the interests of the locating the missing persons," Budapest police said in a statement, adding that the river was flowing at speeds of 9-11 kilometers (5.6-6.8 miles) per hour.

A crisis management team from the South Korean government arrived in Budapest late Thursday, as well as representatives from the South Korean tourist agency which organized the ill-fated trip.

"The most important emphasis of our government is the protection of Korean nationals overseas," Ministry of Foreign Affairs Secretary Oh Sai Juengh said upon his arrival in Budapest.

The sunken boat was located early Thursday near the Margit Bridge, not far from the neo-Gothic Parliament building on the riverbank.

Video released by Hungarian police showed the sightseeing boat, identified as the Hableany (Mermaid), traveling closely side by side and in the same direction as a German-built Viking cruise ship as they approached the bridge Wednesday night.

The Hableany then appeared to steer slightly to its left, into the path of the 135-meter (443-feet) long cruise ship, which continued to sail on at the same speed. The two collided and the sightseeing boat was then seen tipping on its side between the bridge’s two supports.

"As the Viking comes into contact with (the Hableany), it overturns it and in about seven seconds, as it turn on its side, it sinks," Police Col. Adrian Pal said.

Pal said it's unclear what caused the Hableany to steer into the path of the Viking. He said several people aboard the Hableany fell into the water after the collision. The South Korean government said none of those on board was wearing a life jacket.

Police said rescue operations were hampered by the rain and the fast flow of the rising Danube. The search for the 21 missing extended far downstream, even into Serbia, where the Danube goes after leaving Hungary.

Hungarian authorities search for remains from a tourist boat that capsized on the Danube river the night before, killing at least seven people, on May 30, 2019 in Budapest, Hungary. David Balogh / Getty Images

The river, which is 450 meters (500 yards) wide at the point of the accident, was fast-flowing and rising as heavy rain continued in the city. Water temperatures were about 10 to 12 degrees Celsius (50-53 Fahrenheit).

Hajoregiszter.hu, a local ship-tracking website, lists the Hableany as having been built in 1949 in the former Soviet Union. The Viking Sigyn was built this year, according to cruisemapper.com.

CCTV footage recorded on Wednesday night showed that the river was busy with boats of different sizes traveling in both directions.

Budapest has enjoyed a boom in overseas tourism in recent years. Long-haul flights from as far away as Dubai and Beijing increasingly fly visitors from Asia and the Middle East to the Hungarian capital, a relatively affordable but history-rich European destination.

Earlier, the news website Index.hu said one of those rescued was found near the Petofi Bridge, about 3 kilometers (2 miles) south of Parliament.

Police have questioned the Ukrainian captain of the Viking ship, but no details have been released. Authorities said both vessels’ captains are experienced, with many years of service with their companies. South Korean officials said the tourists were not wearing life jackets.

Relatives and a team of 25 official rescue workers were on their way from South Korea to Budapest Thursday. The team of fire, navy and coast guard workers includes some who had been involved in rescue operations for the 2014 South Korean ferry disaster that killed more than 300 people — one of the country’s worst maritime accidents.

Seoul’s presidential Blue House said President Moon Jae-in has called Hungarian Prime Minister Viktor Orban to discuss rescue operations. Moon thanked Orban for the Hungarian government’s rescue efforts, and requested further support for efforts to find the missing, treat the survivors and recover bodies.

The Blue House said Orban told Moon that more than 200 divers and medical staff are involved in the rescue efforts and that officials are planning to locate and hoist the ship.

Employees from the South Korean Embassy in Budapest were assisting Hungarian officials in identifying those rescued and the deceased.

Budapest’s Disaster Management Office Chief Col Zsolt Gabor Palotai said the Hungarian army is setting up a pontoon near the capsized ship’s wreck and divers will go into the Danube from there.

The Very Good Tour agency said the tourists left South Korea on May 25 and were supposed to return June 1.