STOCKHOLM (AP) — A key witness to the murder in Congo of two U.N. experts was poisoned in custody, Swedish broadcaster SVT reported Wednesday, citing a new confidential report by the United Nations.

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.

American Michael Sharp and Sweden's Zaida Catalan were killed in March 2017, while investigating possible violations of U.N. sanctions against Congo.

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 U.N. 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 U.N. 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.

AP writer Jan M. Olsen contributed to this story from Copenhagen, Denmark.