Four bodies were recovered as the Hungarian police on Tuesday began lifting a river cruise boat that sank on Danube river in Budapest last month, BBC reported. The accident had claimed 20 lives with eight people still missing.

Divers entered the sightseeing boat as a floating crane slowly brought it to the surface. The lifting operation by Hungarian and South Korean teams was hampered due to the strong current in swollen Danube river after weeks of rain.

Footage from the site of recovery showed the cabin and upper deck of the boat emerge from the water and divers then carry out a search for victims still trapped inside. Pumps were used to remove water from the hull and the rest of the tourist boat was slowly brought to the surface.

A police spokesperson told AFP that the operation could take six hours, and will be phased to allow divers to search the vessel for any victims still inside.

The bodies recovered were believed to be those of three South Koreans and one of the Hungarian captain of the boat, according to South Korea’s Yonhap news agency.

On May 29, a river cruise boat with 35 people on board capsized and sank seconds after colliding with another vessel on Danube river. Only seven people of the 35 are known to have survived the accident with others missing. More bodies were recovered in the following days of the accident.

The accident happened on a part of the Danube river popular for pleasure trips. The boat was found near the Marguerite Bridge, which connects the old city of Buda and the district of Pest afterwards. Mermaid was a 27-metre (89-feet) double-decker river cruise boat with a 150-horsepower engine and had a capacity to hold up to 60 people.

Police have launched an investigation into the cause of the accident and the captain of the river cruise boat which collided, the Viking Sigyn, has been arrested on suspicion of “endangering waterborne traffic resulting in multiple deaths”.

South Korean President Moon Jae-in had also instructed officials to “dispatch a rapid response team to Hungary immediately.” He also asked to mobilise every possible means to help support rescue efforts in cooperation with the Hungarian government.