"The General Department of Criminal Investigation of Benghazi, eastern Libya, has found a mass grave in Sirte. It contained the remains of five martyrs," Benghazi's General Administration of Criminal Investigation said in a statement.
"Three of the bodies were in military uniform and showed signs of being shot. The other two bodies were decapitated, meaning that they were slaughtered by IS. DNA samples were taken to identify the bodies," according to the statement.
Sirte, 450 km east of the capital Tripoli, was once dominated by IS militants. Forces allied with the UN-backed government, which mostly come from the nearby city of Misrata, took over the city and expelled the terrorists in December 2016.
Earlier in January, the east-based army of Libya announced taking over Sirte and expelling the forces of the rival UN-recognized Government of National Accord (GNA) led by Fayez al-Sarraj.
The east-based army under General Khalifa Haftar has been leading a military campaign since April 2019 in and around Tripoli, attempting to take over the city and topple the GNA.
Thousands have been killed and injured in the fighting while hundreds of thousands of civilians left their homes to escape the violence.