The captain of a vessel that collided with a small tour boat on the Danube in Budapest, resulting in at least seven deaths, has been arrested, police say.
The 64-year-old Ukrainian national was held as a suspect over reckless misconduct in waterborne traffic leading to mass casualties.
The boat carrying South Korean tourists sank seven seconds after the collision during a rainstorm on Wednesday.
Hopes have faded that any of the 21 people missing will be found alive.
A criminal investigation has been launched into the collision, a rare incident on the Danube where navigation is busy but generally safe.
Seven people rescued alive were suffering from hypothermia but stable, an ambulance spokesman said.
South Korea's foreign minister, Kang Kyung-wha, is due to arrive in Budapest on Friday for a joint press conference with her Hungarian counterpart Péter Szijjártó.
Witness saw people 'screaming in the water'
CCTV footage showed the boat that sank - Hableany, or Mermaid - being hit from behind by a larger tour boat - Viking Sigyn - near the Margit (Margaret) Bridge in central Budapest after 21:00 local time (19:00 GMT).
"The whole thing happened very quickly" said Clay Findley, a US tourist who was on the Viking Sigyn.
"I thought at first we were going to miss it, but the front of the Viking hit the back of that little boat... and then the hull popped up on the opposite side of the ship, just a few seconds later, and then it was down."
Another passenger, 66-year-old Ginger Brinton from the US, told the AFP news agency that he was on the boat's balcony when he "saw people in the water, screaming for help".
He added: "We never felt any bump. We didn't realise. We just saw people in the water. It was just terrible."
No-one on the Swiss-registered Viking Sigyn was injured.
Hungarian police said the investigation yielded evidence that raised questions of personal responsibility so it questioned the captain - identified as Yuriy C., from Odessa - as a suspect and later decided to take him into custody.
Emergency crews found the wreckage of the Hableany, a double-decker river cruise boat built in 1949 in the former Soviet Union, on the riverbed near the Margaret Bridge and were preparing to lift it.
What is known about the rescue operation?
Thirty South Korean tourists and three tour guides, as well as two Hungarian crew, were on board the Hableany.
Most of the tourists were aged between 40 and 50 but the group also included a six-year-old child and a man in his 70s, South Korea's Yonhap news agency reported.
The seven rescued alive included a 31-year-old passenger identified only by her surname Jung, who told Yonhap: "The current was so fast and people were floating away."
Another survivor, a 32-year-old woman named as Yoon, said: "The boat flipped instantly and capsized."
The rescue operation continued with boats, divers, spotlights, and radar scanning along the river, where the temperature of the water was between 10C and 12C (50F-54F).
But teams warned that, as more time passed, the strong currents caused by heavy rainfall would carry people further downstream, lessening the chances of finding survivors.
The seven confirmed victims were not wearing life jackets and three of them were found several kilometres from the site of the collision, police said.
'An accident waiting to happen'
By Nick Thorpe, BBC News, Budapest
There has been a dramatic increase in river traffic in recent years as tourism increases in Budapest.
Andras Kurbely, a crew member of another large cruise ship with 27 years of experience on the Danube, told the BBC that this was an accident he and others have long worried about.
"It's just not healthy to have so many large ships, which are much more powerful and harder to manoeuvre, carving a route between so many smaller boats."
In his view, the practice of the river cruisers, which sail up and down the river between the five main bridges as an after-dinner excursion for their passengers, should be stopped.
What has the reaction been?
Relatives of the passengers were travelling from South Korea to Hungary.
Meanwhile, flowers were laid outside the country's embassy in Budapest and candles were lit on the Danube bank.
Earlier, South Korean President Moon Jae-in said his government planned "to co-operate with the Hungarian government to thoroughly investigate the cause of the accident".
For South Koreans, the sinking is a painful reminder of the Sewol disaster in 2014, the BBC's Laura Bicker in Seoul reports.
The ferry of that name sank off South Korea's Jindo island, killing 304 people, almost all of them schoolchildren on a trip. The ship's captain was later convicted of murder.