Dozens of bodies, apparently of Gaddafi loyalists, some of whom may have been executed by revolutionary forces, have been discovered in the Libyan town of Sirte.
Human Rights Watch said that the discovery of 57 corpses in Gaddafi's hometown "seems part of a trend of killings, looting and other abuses committed by anti-Gaddafi fighters who consider themselves above the law."
The group urged Libyan authorities to rein in armed groups.
The latest discovery came as Libya's new leaders declared the country liberated, following a brutal eight-month civil war. The declaration was overshadowed by continued questions about whether Gaddafi was executed after capture last week.
The US and Britain have called for an investigation and Libya responded by ordering an inquiry into Gaddafi's death.
Transitional leader Mustafa Abdul-Jalil said that the National Transitional Council had formed a committee to investigate the killing amid conflicting reports of how the dictator died.