Military prosecutors in DR Congo say they have charged a key "witness" with conspiracy to murder over the 2017 killing of two UN experts in the war-torn central Kasai region.
The experts -- Zaida Catalan, a 36-year-old Swedish-Chilean national, and American Michael Sharp, 34 -- were investigating mass graves associated with a rebellion on behalf of the UN Security Council when they were abducted and killed on March 12.
Kinshasa initially blamed the murder on members of the Kamwina Nsapu militia, who embarked upon a campaign of armed violence after troops killed their leader in August 2016.
But there have been growing questions about the possible role of state agents in organising the attack, which have intensified with the arrest and indictment of Jean Bosco Mukanda, a local teacher who used to be a militia leader.
Initially, when the case first went to trial in June 2017, prosecutors identified Mr Mukanda as a key witness who had allegedly arrived at Moyo Musuila village shortly after the pair had been killed.
He supplied important details to UN investigators and also identified several people who are now the main suspects in the murder case before the trial was suspended in October that year.
When the trial resumed last August after the arrival of Canadian lawyer Robert Petit to assist the investigation aided by a UN regional oversight mechanism, Mr Mukanda turned from witness to suspect after evidence suggested he was actually a military informer and not the innocent bystander he had claimed to be.
Arrested in September, he was indicted this week on charges of conspiracy to murder and murder with mutilation at the military court in the city of Kananga, where the trial is being held.
The decision to indict him came after a close examination of his phone records showed frequent calls between him and many of the main suspects in the murder, according to UN-backed broadcaster Radio Okapi's website.
Several defendants have also claimed he played an active role in the deaths of the two UN experts.
"We understood that this was a man who took part in everything that happened," the prosecutor said this week.
Mr Mukanda's indictment was hailed by the defence team as a major coup.
"We said Mukanda knew too much and that he ought to be a defendant. He knew the ins and outs of everything that happened at the scene of the crime, about the preparations in Kananga and Moyo Musuila," said lawyer Tresor Kabangu.
"He even knew what happened after the experts were killed."
The Congolese army has denied involvement in the murder although one of its officers, Colonel Jean de Dieu Mambweni, was arrested in December on suspicion of preparing the mission that led to the killings.
A total of 22 people are to be tried -- Constantin Tshidime, the head of Moyo Musuila village where they were killed, and a militia leader called Vincent Manga.
In 2017, a UN panel concluded that the pair were likely killed during a random ambush by members of a militia group, while refusing to rule out any involvement of the authorities.
But an investigation by Radio France Internationale (RFI) and Reuters the same year implicated three state agents in the "organisation" of the attack.
And three months ago, a joint investigation by five international news organisations suggested that the UN had "buried evidence" suggesting the Congolese authorities may have had a role in the murder.
Investigators on a UN task force working independently from the panel have complained they have been routinely misled by the Congolese authorities, accusing them of making little effort to interview key witnesses while tampering with others.