The report, released on Thursday, looks at the period preceding and following the May election of retired General Evariste Ndayishimiye, a member of the governing party who came to power after the 15-year rule of Pierre Nkurunziza.
Nkurunziza, who became president in 1995 after the country's 12-year civil war, was elected for a third term in 2015 in a poll largely boycotted by the opposition.
At least 1,200 people were killed and more than 400,000 displaced during the ensuing unrest between April 2015 and May 2017, according to the UN.
Al Jazeera's Alexi O'Brien reports.
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