Seventeen human skulls have been found in a wooded area in a suburb of the Haitian capital Port-au-Prince.
The skulls have been taken away for forensic examination
Witnesses said at least some of the skulls were thrown from a moving car in Petionville on Saturday morning.
The UN serious crimes unit is helping local police to investigate the find, a UN official said.
Haiti has been hit by a wave of violence and kidnappings since the rebellion that toppled former President Jean-Bertrand Aristide in 2004.
The grim discovery was made on Saturday, in a rubbish-strewn area of woodland in Petionville suburb, not far from several upmarket restaurants.
Some of the skulls were found in small grey plastic buckets.
'Could be homicide'
Local resident Emmanuel Crepsac, 44, told the Associated Press bystanders had seen an object thrown into a roadside embankment from a moving car.
"They said someone in the car just threw something down there and then sped off. When people went to see what it was, they saw some skulls," he said.
UN civilian police officers were measuring and numbering the skulls before removing them, an AP reporter witnessed.
"All we know is that 17 skulls have been discovered," said UN spokesman David Wimhurst.
"The serious crimes unit (of the UN) is assisting the Haitian National Police, who are leading the inquiry."
"It could be homicide," Haitian police spokesman, Frantz Lerebours, said, adding that forensic tests would be carried out on the skulls.