“Our investigation targets Felix Sturm, his manager, and one other male person,” prosecutor Benedkit Kortz said, confirming a report in Bild.
Sturm and his manager Roland Bebak are believed to have hired the third man to kill promoter Ahmet Öner, he added.
Öner had previously organized bouts for the boxer, but the two parted ways after a serious falling-out.
“This is a far-fetched story, it's false and contrived,” Bebak said through a spokesman.
“Such an accusation isn't a joke, but deadly serious,” he added.