It is believed that the migrants wanted to make their way north through Nicaragua and on to the United States. But it is uncertain whether the migrants were African or Haitian.
The bodies of eight undocumented migrants have been found in Lake Nicaragua, according to police.
"They died by drowning. We believe they were Africans, going by the color of their skin," said Mirian Rugma of the Rivas regional police force.
Earlier the police issued a statement announcing the discovery of the bodies.
"Given their physical characteristics and their clothes, it is presumed they were people of African origin trying to enter the United States," it said.
Autopsies determined they had drowned.
But experts in migration patterns through Central America say that recently many black migrants crossing through the region are in fact Haitians passing themselves off as Africans in the hope of gaining asylum in the United States.
The bodies of seven men and one woman, who appeared to have been pregnant, were found between Sunday and Tuesday floating near the southern shore of Lake Nicaragua, close to the town of Cardenas.
They were just two miles (3 kilometers) from the Costa Rican border.
Rugma said police believe the victims entered the country by boat, coming up a cross-border river that feeds into the lake.
Nicaragua had tightened security along its southern border with Costa Rica last November to keep out undocumented migrants looking to head north towards the US.
As a result, there are now more than 2,400 migrants camping out in tents, schools and various shelters in Costa Rica, while they search for a way into Nicaragua.
The International Organization for Migration says those trying to make their way north, through Nicaragua, include Africans but also migrants from the Caribbean nation of Haiti.
But others, about 10 percent, are from Pakistan and Afghanistan. Having found their path into the European Union blocked, they are attempting to seek asylum in the US.
Some of the stymied migrants are turning to human smugglers who charge more than $1,000 each.
Crossing the vast lake by boat appeared to be their last option.
bik/bw (AFP, AP, dpa)