Insecticide was found in the stomach of the witness who was found dead in his military cell in Congo Oct. 23, said SVT. The broadcaster didn't name the man, but others have identified him as Tshikele Kengayi.
Congo has blamed the killings on the Kamwina Nsapu militia active in the central Kasai provinces. However, rights groups have said security forces may have been involved, which the government denies.
The UN report also says the Congolese prosecution suspects Col. Jean de Dieu Mambweni is behind the murders, SVT said. He was arrested in early December after failing to disclose that he had met with Sharp and Catalan just two days before they were brutally murdered. The Congolese colonel was remanded in custody.
The report also states a militiaman charged with taking part in the assassination of Catalan and Sharp, worked for the Congolese Army.
The report, dated Dec. 12 and written by Robert Petit, an attorney sent to Congo by the UN secretary-general to assist in the ongoing investigation, said "further testing is planned to confirm if the substance was in lethal quantity," according to SVT.
Suicide by taking insecticide has been reported in Africa before and a Zimbabwe cabinet minister killed himself in the 1980s by ingesting rat poison.
With inputs from agency.