A mass grave containing 53 decaying bodies has been discovered in Sirte, a human rights group has said.
Human Rights Watch (HRW) said the discovery suggested some of Colonel Muammar Gaddafi's loyalists were killed.
Peter Bouckaert, from the organisation, said: "We found 53 decomposing bodies, apparently Gaddafi supporters, at an abandoned hotel in Sirte.
"Some had their hands bound behind their backs when they were shot," he added.
An investigator from the group found the bodies on Sunday at the Hotel Mahari in Sirte, an "area of the city that was under the control of anti-Gaddafi fighters from Misratah before the killings took place," a statement from HRW said.
"The bodies were clustered together, apparently where they had been killed, on the grass in the sea-view garden of the hotel," the statement added.
The group said the killings appeared to be part of a trend of slayings, looting and other abuses committed by anti-Gaddafi fighters who considered themselves above the law.
It implicated several groups of fighters from Misratah, who were said to have held the hotel from before the time of the killings until October 20 when the fighting stopped.
Graffiti on the walls of the hotel showed the names of five Misratah-based groups, HRW said.
HRW has now urged Libyan authorities to clamp down on armed groups.
"If the National Transitional Council (NTC) fails to investigate this crime it will signal that those who fought against Gaddafi can do anything without fear of prosecution," the group said.
The group also discovered the decaying bodies of 10 people near a water reservoir, but has been unable to determine their identities.