Four senior Burundi police officers have been sentenced to death for the 2001 killing of the World Health Organization country representative.
Manlan's body was found on the shore of Lake Tanganyika
The body of Kassi Manlan, from Ivory Coast, was found by fishermen on the shore of Lake Tanganyika.
The five judges did not say why Manlan was killed but a defence lawyer told the court that Mr Manlan had been investigating corruption allegations.
Nine others were given jail terms for their parts in the murder.
Those sentenced to death were a former chief of security police, the ex-deputy director of public intelligence services, a former director of Burundi's main jail and a former top intelligence officer at the mayor's office.
They were found guilty of planning the murder, while some of those given jail terms carried out the killing.
One of the defence lawyers said that the trial should continue until the motive for Manlan's murder was established.
He said that Mr Manlan had making enquiries into reports that money meant to help Burundi fight malaria had been embezzled by government officials.
"I have never been told why my husband was killed," Mr Manlan's wife said from her home in Ivory Coast.