WASHINGTON -- Rebels who ambushed Libyan leader Moammar Gadhafi’s convoy not only probably killed the strongman and his son after capture but also summarily executed more than 60 of his supporters in a nearby hotel, according to the most detailed examination to date of Gadhafi’s grisly last moments after he fell into the hands of U.S.-backed rebel forces one year ago Saturday.

The report, by the advocacy group Human Rights Watch, calls the deaths “among the most clear-cut instances of extrajudicial executions carried out by opposition fighters.” The report, which was given to reporters in advance, is to be released Wednesday morning in Beirut.

Executing captives is a war crime, but Human Rights Watch blasted the transitional Libyan government for failing to conduct even a cursory investigation into the well-documented killings. The government’s failure to investigate, the report said, “shows that the new Libyan authorities have a long way to go to make their professed commitment to the rule of law and ending impunity for human rights abuses a reality.”

How Libyan authorities respond to the apparent violation also could signal how seriously they’ll take other high-profile open cases, most notably the hunt for the assailants whose attack on the U.S. consulate in Benghazi last month killed the U.S. ambassador and three other Americans. While Libyan authorities have said some people have been arrested in relation to the case, little information has emerged about their identifies or what charges they might face.

Libyan officials didn’t respond to McClatchy’s requests for comment. The Human Rights Watch report says that Libyan officials responded to the group’s request for comment with a letter that dwelled on crimes of the former regime and barely mentioned the killings.

“The peaceful demonstrators – who became revolutionaries in self-defense – took special care to arrest Moammar Gadhafi alive, in order to try him in a fair trial for the crimes he committed against his own people,” the letter said.

Gadhafi and his fifth son, Mutassim, both died on the day of their capture, Oct. 20, 2011. Opposition forces detained an estimated 150 people alive after the battle and transported 70 of those to the city of Misrata to be held. But 66 of those were found dead the next day at a local hotel, according to video footage, witness testimony and firsthand observations Human Rights Watch compiled in its investigation of the rebels’ atrocities.

“This report presents evidence that Misrata-based militias, after capturing and disarming members of the Gadhafi convoy and bringing them under their total control, subjected them to brutal beatings before apparently executing dozens of them,” Human Rights Watch said. “Seven months later, Libyan authorities have neither investigated nor held accountable those who committed these crimes.”

The report provides the first detailed accounting of what happened to Gadhafi after NATO-backed rebel forces routed government forces from the Libyan capital, Tripoli, in August 2011.

According to the report, Gadhafi, Mutassim and a coterie of insiders eventually took shelter in Gadhafi’s besieged hometown of Sirte, a coastal city halfway between Tripoli and Benghazi. A witness interviewed while in detention told Human Rights Watch that conditions were so miserable under the siege that Mutassim Gadhafi, who was accustomed to a prince’s lifestyle, grew short-tempered over the lack of electricity.