MEXICO CITY—International investigators invited by Mexico to help solve the disappearance and probable killing of 43 college students nearly two years ago released a scathing 608-page report detailing how constant obstacles put up by the Mexican government, and the widespread use of torture in criminal investigations, sabotaged their probe.

The investigators said Sunday that evidence showed Mexico’s federal police and the police of Guerrero state had a role in the disappearances of the students that night—an incident that...