Dozens of bodies Colonel Gaddafi loyalists - some of whom may have been executed by rebel forces - have been discovered in the Libyan town of Sirte.
Human Rights Watch said the discovery of 57 corpses in Muammar 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.
Peter Bouckaert, emergencies director of Human Rights Watch, said: "The evidence suggests that some of the victims were shot while being held as prisoners, when that part of Sirte was controlled by anti-Gaddafi brigades who appear to act outside the control of the NTC. If the NTC fails to investigate this crime it will signal that those who fought against Gaddafi can do anything without fear of prosecution."
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 his capture last week.
Libya's interim leader has ordered an investigation into Gaddafi's death after strong international pressure.
It will aim to determine how he was killed by a bullet to the head shortly after he was captured alive.
Mustafa Abdul-Jalil said the National Transitional Council has formed a committee to investigate the killing amid conflicting reports of how Gaddafi died.
Government officials said initial findings suggest he was killed in crossfire as his supporters clashed with revolutionary forces seizing control of his hometown of Sirte.
But Mr Abdul-Jalil raised a new possibility, suggesting that Gaddafi could have been killed by his own supporters to prevent him from implicating them in past misdeeds under his regime.
"Let us question who has the interest in the fact that Gaddafi will not be tried. Libyans want to try him for what he did to them," he said. "Free Libyans wanted to keep Gaddafi in prison and humiliate him. Those who wanted him killed were those who were loyal to him or had played a role under him, his death was in their benefit."
Defence Secretary Philip Hammond has revealed that 90 British soldiers were involved in the Nato operation in Libya. The troops were among 1,200 UK service personnel sent abroad as part of Operation Ellamy, the Ministry of Defence's codename for the Libya conflict. Previously, ministers claimed "a handful" of British Army officers would be sent to "advise" rebel commanders plotting the overthrow of Gaddafi. Mr Hammond revealed the latest figures in a written Parliamentary answer to Labour MP Jeremy Corbyn. The Defence Secretary said 350 Royal Navy seamen and 700 RAF airmen were sent to Libya.