Exhumation teams unearthed “42 unidentified bodies” after following up reports of a “mass grave” at the site of a former school in Sirte, the authority said.
“DNA samples have been taken for analysis in coordination with the office of forensic medicine,” it added, without elaborating further.
Sirte, a central coastal city, was held by IS between 2015 and 2016, as it exploited the chaos engulfing much of Libya in the wake of the 2011 overthrow and killing of dictator Moamer Kadhafi in a NATO-backed uprising.
The jihadist group was dislodged by forces loyal to the then Government of National Accord in December 2016 after months of intense house-to-house fighting.
In October 2017, a grave containing the bodies of 21 Coptic Christians, executed by the jihadists two years earlier, was uncovered near the Mediterranean city.
Another mass grave with the remains of 34 Ethiopian Christians was discovered near Sirte in December 2018, more than three years after IS published a video showing its personnel executing at least 28 men described as Ethiopian Christians.