Fourteen bodies have been found in the east African nation of Burundi, in a river west of the capital, Bujumbura. At least one was decapitated.
The BBC reports the brutality of the killings is feeding fears that a new rebellion is brewing in the country, which is still recovering from a 12-year civil war.
Seven people were killed in an attack last week, which the government has blamed on bandits, not rebels.
Elections earlier this year were undermined after opposition candidates refused to stand against President Pierre Nkurunziza, alleging fraud.
Main opposition leader, former rebel leader Agathon Rwasa, also disappeared before the June polls, and is thought to have fled across the border into the Democratic Republic of Congo.
The BBC reports bodies have been washing up on the banks of the River Ruzizi, close to the border with DR Congo, for the past week.
It appears they were tied up before being thrown into the river.
A government spokesman said the corpses may be linked to an attack last week in the northern Rukoko area, which they believed was caused by bandits targeting farm workers and cattle.
"The government is resolved to track them down so that justice can apply to them a punishment equal to their crimes," he said.
Some 300,000 people died in Burundi's ethnic-based civil war.