Ferry passengers who didn't drown were mauled to death by crocodiles when a driver of a South African truck ignored signs to stop.
Up to 90 people were feared dead on Thursday after the truck - overloaded with cobalt - plunged into the crocodile-infested Zambezi River, sweeping passengers along with it on Wednesday night.
An environmental disaster was also feared after concerns that the cobalt might have leaked into the water.
The truck, believed to be registered in Limpopo, was said to have been carrying cobalt from Zambia's Copper Belt mining area to South Africa when it rolled off the Kazungula ferry between Zambia and Botswana.
Police and military sources said "at least 90 passengers were missing, presumed drowned". There were 10 survivors.
South Africa's High Commission counsellor in Lusaka, Rudi Appel, said reports indicated that 15 bodies had been pulled out of the river by Thursday afternoon.
"It appears as if the truck was from Limpopo, but the owners have not been identified. We have heard the driver of the truck died, but at this point we do not know whether there are any South African victims. I suspect it will take a while before we will receive that information."
Zambia's Works and Supply Minister, Ludwig Sondashi said that, after clearing customs, the driver of the truck had failed to respond to signals to stop. "The driver just flashed his lights and continued to drive onto the pontoon and lost control."
There was huge confusion on the Zambian side of the river when the truck came loose and rolled over the side, almost capsizing the boat, which it was estimated was carrying at least 100 passengers.
Many passengers were on the deck next to the truck and were flung into the river - they drowned or were attacked by crocodiles. Some victims drowned under the upside-down truck on the riverbed.
Police and military personnel from Zambia and Botswana started a race against time to retrieve the truck and the bodies.
Kazangula police station commander Superintendent Frankel Mohutsiwa said it appeared the Zambian authorities did not keep a register of passengers, making it difficult to assess the actual number of people on board.
A search of the river was being undertaken by Botswana Defence Force divers and the Zambian army, assisted by villagers who feared their relatives could have been aboard.
Raban Chanda, head of the University of Botswana's environmental science department, said the disaster should "act as a prompter" to the building of a planned bridge linking Botswana and Zambia.
Professor Lavinder Ahmed, of the department of chemistry at the university, said he did not think the cobalt would contaminate the river.
"Cobalt tends to rust in water and as the river flows strongly, this will not be an immediate danger."
It was not clear why the shipment was going to South Africa. Spokesperson for the department of foreign affairs, Ronnie Mamoepa, said the origin of the truck and the nationalities of the victims were being investigated.