BUENOS AIRES—Investigators here have concluded that prosecutor Alberto Nisman was murdered in 2015 hours before he was to testify in Congress against then-President Cristina Kirchner, according to people familiar with the investigators’ report, opening a new chapter in a mystery that has transfixed Argentina.

The report was submitted to a federal court on Friday. The probe is part of an effort to resolve a case that has become a milepost in a long history of unresolved, high-profile allegations against the country’s political...