Libya's interim rulers say they have found a mass grave believed to hold the remains of 1,270 inmates killed by Moammar Gadhafi's security forces in a notorious 1996 massacre at Tripoli's Abu Salim prison.

Investigators found the site, a field scattered with bone fragments, near the prison walls, and using information obtained from witnesses and former Gadhafi officials.

Most of the inmates killed were political prisoners, including Islamic clerics and students who had dared to speak out against Mr. Gadhafi. In June 1996, they rose up to protest conditions at the facility and were gunned down by forces directed by some of the former leader's inner circle.

Lingering anger over the massacre ignited the uprising against Mr. Gadhafi in February, when families of the inmates killed at Abu Salim demonstrated in the eastern city of Benghazi to demand the release of their lawyer.

Elsewhere in Libya, NATO warplanes pounded Mr. Gadhafi's hometown of Sirte on Sunday with an unusually heavy bombardment. NATO said alliance forces were bombing loyalists in Sirte after reports emerged of “executions, hostage-taking and the targeting of individuals within the city.”

Fighters for the National Transitional Council earlier pushed into the town center but met stiff resistance from pro-Gadhafi forces. The fighters later withdrew to clear the way for the NATO airstrikes.

Also Sunday, pro-Gadhafi gunmen, including some who crossed the border from Algeria, attacked revolutionary forces in a frontier town, killing six people.

An NTC military spokesman said the attack on Ghadames, about 450 kilometers southwest of Tripoli, was carried out by fighters belonging to a unit that had been under the command of Mr. Gadhafi's son, Khamis, who was reported killed in earlier clashes.

Mr. Gadhafi's whereabouts remain unknown.

On Friday, his daughter Aisha released an audio recording in which she said her father is in high spirits and is fighting alongside supporters.

The NTC has said an interim Libyan government will be announced this week. NTC chairman Mustafa Abdel Jalil spoke Saturday in Benghazi, where the NTC has been meeting to discuss the new government's formation.

Jalil said two seats would be reserved for representatives from Sirte. A spokesman said earlier the government would include 22 ministers and a deputy prime minister.