Libyan strongman Moammar Gadhafi may not have been killed in crossfire during his capture, but instead executed along with 66 others, including one of his sons, according to a new report.

Human Rights Watch says new evidence it collected about what happened one year ago "implicates Misrata-based militias in the apparent execution of dozens of detainees" after Gadhafi's death.

In an extensive report, "Death of a Dictator: Bloody Vengeance in Sirte," the human rights group says Libyan authorities have failed to follow through on a vow to investigate the death of Gadhafi, his son Mutassim, and dozens of others in rebel custody.

"Among the most powerful new evidence is a mobile phone video clip filmed by opposition militia members that shows a large group of captured convoy members in detention, being cursed at and abused. Human Rights Watch used hospital morgue photos to establish that at least 17 of the detainees visible in the phone video were later executed at the Mahari Hotel," the group says.

A Human Rights Watch research team visited the site of the final battle between Gadhafi's convoy and opposition forces, and interviewed officers in opposition militias as well as surviving members of the convoy.

"Our findings call into question the assertion by Libyan authorities that Moammar Gaddafi was killed in crossfire, and not after his capture," said Peter Bouckaert, emergencies director for Human Rights Watch.

CNN's timeline of Gadhafi's final moments notes that as rebel forces were putting him into a vehicle, a firefight erupted and, caught in the crossfire, Gadhafi was shot in the head. He died moments before arriving at a hospital, according to then-interim Prime Minister Mahmoud Jibril.

Human Rights Watch says video footage showed Gadhafi was "captured alive but bleeding heavily from a head wound, believed to have been caused by shrapnel from a grenade thrown by his own guards that exploded in their midst, killing his defense minister, Abu Bakr Younis." In the footage, Gadhafi is "severely beaten by opposition forces and stabbed with a 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."