International Criminal Court prosecutors are appealing a decision to free Congolese ex-militia boss Mathieu Ngudjolo Chui for war crimes last year, asking the court to order a new trial.
"The prosecution requests the Appeals Chamber... to reverse the judgement... and remand the case for a re-trial before a different trial chamber," ICC chief prosecutor Fatou Bensouda said in a court document, dated Wednesday and made available on Friday.
Ngudjolo, 42, is a former leader of the Nationalist Integrationist Front (FNI) militia in the eastern Democratic Republic of Congo (DRC) and his acquittal on December 18 was a first for the world's only permanent war crimes tribunal.
Judges said the prosecution failed to prove Ngudjolo's commanding role in a 2003 attack on Bogoro village in the vast African country's northeastern Ituri region, in which more than 200 civilians were slaughtered.
Ngudjolo faced charges of using child soldiers in the ethnically-motivated attack in which many of the Hema victims were burnt alive, babies smashed against walls, women raped and taken away as sex slaves.
Chief prosecutor Bensouda, in the appeal, said the judges did not properly weigh up the evidence, including by a UN human rights investigator who testified that Ngudjolo admitted to her that he organised the Bogoro attack.
Although judges found this witness to be credible and "thus accepted that Ngudjolo admitted to her that he organised the attack," they declined to accept his confession as a proof of guilt, Bensouda said.
"This wholly speculative and factually unsupported finding -- contrary even to the testimony of Ngudjolo himself -- illustrates the Chamber's flawed approach to the evidence that, ultimately, led it to acquit Ngudjolo."
A former deserter from the old Zaire army (FAZ) in 1996, Ngudjolo later became a leader of the Lendu-based FNI and "had ultimate control over FNI commanders", prosecutors alleged.
His main adversary, Hema former militia leader Thomas Lubanga, was in July jailed for 14 years for using child soldiers in his own rebel army.
In 2003, DR Congo was just starting to emerge from a war that had embroiled the armies of a half-dozen nations, and the isolated east was rife with violent militia groups.
Clashes in Ituri broke out in 1999 and devastated the region, said the indictment, leaving some 60,000 dead, according to non-governmental group tallies.
The December verdict disappointed human rights groups, saying it left victims of the Bogoro attack "lacking justice."