(CNN) -- The bodies of 53 people, believed to be supporters of ousted Libyan leader Moammar Gadhafi, has been found in a hotel that was under the control of anti-Gadhafi fighters, Human Rights Watch said Monday.
The rights group said it found the bodies clustered together at Hotel Mahari in Sirte on Sunday. About 20 residents were putting the bodies in body bags to prepare them for burial when Human Rights Watch found them.
"We found 53 decomposing bodies, apparently (Gadhafi) supporters, at an abandoned hotel in Sirte, and some had their hands bound behind their backs when they were shot," said Peter Bouckaert, emergencies director of Human Rights Watch.
"This requires the immediate attention of the Libyan authorities to investigate what happened and hold accountable those responsible."
Officials with the National Transitional Council, Libya's new leadership, were not immediately available for comment.
The residents told Human Rights Watch investigators they found the bodies last week after the fighting in Sirte stopped and they returned home.
They identified some of the deceased as Sirte residents and Gadhafi supporters.
Sirte, Gadhafi's hometown, was one of the last cities to fall before the National Transitional Council declared the country liberated Sunday following Gadhafi's 42-year rule.
However, anti-Gadhafi fighters from Misrata had controlled the area of Sirte where the hotel is located since early October, Human Rights Watch said, citing witnesses.
On the entrance and walls of the hotel, the group said it saw the names of several brigades from Misrata.
Based on the condition of the bodies, the group's investigators determined the 53 had been killed between October 13 and 19.
"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 National Transitional Council," Bouckaert said.
"If the NTC fails to investigate this crime it will signal that those who fought against Gaddafi can do anything without fear of prosecution."
At a different location, Human Rights Watch found 10 decomposed bodies that had been dumped in a water reservoir. The group could not determine whether the bodies belonged to pro- or anti-Gadhafi forces.
In addition, medical officials in Sirte told the group that pro-Gadhafi forces had carried out killings in the city and that they had found 23 bound bodies between October 15 and 20.
Meanwhile, questions linger about the circumstances surrounding Gadhafi's death Thursday, with Human Rights Watch and others questioning the Libyan interim government's version.
An autopsy revealed Gadhafi died from gunshot wound to the head.
Mahmoud Jibril, chairman of the NTC's executive board, has said Gadhafi's right arm was wounded when a gun battle erupted between the fighters and Gadhafi loyalists as his captors attempted to load him into a vehicle.
More shooting erupted as the vehicle drove away, and Gadhafi was shot in the head, dying moments before arriving at a hospital in Misrata, Jibril said, citing the city's coroner.
On Monday, Human Rights Watch said it found the remains of at least 95 people who were apparently killed at the site Thursday. Some died in the fighting and in the NATO airstrike on Gadhafi's convoy prior to the ousted leader's capture. But the group said that about six to 10 seemed to have been "executed" -- with bullet wounds to the head and body.
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