Misrata was subjected to a brutal weeks-long siege by Gadhafi's forces that killed hundreds of residents, and fighters from the city became among the regime's most implacable foes. HRW says it seems the Misratans took revenge against their prisoners in Sirte.
"The evidence suggests that opposition militias summarily executed at least 66 captured members of Gadhafi's convoy in Sirte," said Peter Bouckaert, emergencies director at Human Rights Watch.
The New York-based group's report says that new evidence unearthed in its investigation includes a mobile phone video clip taken by militiamen showing a large number of prisoners from Gadhafi's convoy being cursed and abused by opposition fighters.
The remains of least 17 of the detainees in the phone video were later identified in a group of 66 bodies found at Sirte's Mahari hotel, some still with their hands tied behind their back. Human Rights Watch said it used hospital morgue photos to confirm the victims' identities.
The dictator himself was seen alive in a widely-circulated video made public shortly after the battle.
"Video footage shows that Moammar Gadhafi was captured alive but bleeding heavily from a head wound," the HRW report says. But footage showed that he was "severely beaten by opposition forces, stabbed with bayonet in his buttocks, causing more injuries, and bleeding. By the time he is filmed being loaded into an ambulance half-naked, he appears lifeless."
Bouckaert said the group's "findings call into question the assertion by Libyan authorities that Moammar Gadhafi was killed in crossfire and not after his capture."
Gadhafi's son Muatassim was also videotaped alive and in captivity, only to have his body turn up at a morgue in Misrata alongside his father's.
"In case after case we investigated, the individuals had been videotaped alive by the opposition fighters who held them and then found dead hours later," Bouckaert said. "Our strongest evidence for these executions comes from the footage filmed by the opposition forces and the physical evidence at the Mahari hotel where the 66 bodies were found."
Another victim cited by HRW as an example was 29-year-old Ahmed al-Gharyani, a navy recruit from the town of Tawergha. He was seen alive in the phone video as rebels beat him. His body was later found in the hotel and eventually identified by his family.
His hometown, Tawergha, was used as a staging ground by Gadhafi's forces to launch attacks on Misrata, but after rebels broke the siege on Misrata and overran Tawergha, the town's residents fled or were driven out by vengeful rebels.
Suleiman al-Fortia, a member of the dissolved National Transitional Council from Misrata, denied that Gadhafi or his loyalists were executed. "We hoped to arrest Gadhafi alive (to try him). All the killings took place in a crossfire," he said.
But HRW said that "under the laws of war, the killing of captured combatants is a war crime, and Libyan civilian and military authorities have an obligation to investigate war crimes and other violations of international humanitarian law."
The group released its report days before Libya celebrates "liberation day," the anniversary of Sirte's fall on Oct. 23. Since then, the country's new leaders have heavily depended on former rebel militias to secure cities and protect borders in the absence of a strong national army or other government security forces.
Simmering feuds between rival militias and towns continued to challenge the government's hold on the sprawling country.
A militia affiliated with the Defense Ministry shelled on Wednesday the western town of Bani Walid, one of the few remaining strongholds of Gadhafi loyalists.
The town had been allowed to co-exist uneasily with the new government after Gadhafi's fall but the pro-government Libya Shield militia placed it under renewed siege after residents were reported to have tortured and killed one of the rebels who caught the former dictator in Sirte.
Talks to disarm its fighters and hand over the suspects failed. Witnesses and medical officials said at least two people from the town were killed in the shelling, and 18 Libya Shield members were wounded.
Mahmoud al-Mabrouk, a resident of Bani Walid reached by telephone, said Libya Shield fighters have advanced into his district and were now only a few kilometers (miles) from the city center. He said he saw at least two people die near his house, as heavy shelling with mortars and artillery continued into the early evening.
A doctor in the hospital treating the Libya Shield fighters said 18 of them were wounded in the fighting, including two in critical condition. He spoke anonymously as he was not authorized to talk to the media.
Other militias have been implicated in revenge attacks and communal strife, while members of one Islamist militia have been accused of taking part in the attack on the U.S. Consulate in the eastern city Benghazi on Sept. 11 that killed four Americans, including U.S. Ambassador to Libya Chris Stevens.
In the aftermath of Stevens' death, popular resentment surged and thousands took to the streets of Benghazi demanding the dismantlement of the militias. The government has taken over some militia headquarters and appointed military officers to run the groups, and designated some "outlawed" and others "tolerated."
State Department spokeswoman Victoria Nuland said the U.S. is urging Libya to genuinely investigate all claims of war crimes in last year's civil war and to prosecute perpetrators.
"With regard to Gadhafi's death, we have regularly urged the government of Libya to continue to investigate the circumstances," she told reporters. "And it's very important to hold those responsible to account. This is part not only the judicial maturation of Libya, but also part of the ground that they need to plow for national reconciliation."
Nuland said the U.S. is engaged in an extensive dialogue with Libya on all aspects of its judicial system. This involves training lawyers, judges and civil society.
"We need to now support them as they take the next steps on all of these issues," she said.