War crimes judges have approved the opening of a full investigation into alleged crimes against humanity in Burundi, where at least 1,200 people have been killed in unrest since 2015, court officials revealed Thursday.

The judges found there was "a reasonable basis to proceed with an investigation in relation to crimes against humanity" in a decision made on October 25 -- two days before Burundi became the first nation to leave the International Criminal Court -- but kept under seal until Thursday.