Tripoli, Libya (CNN) -- Horrifying glimpses of the brutality underpinning the rule of Libyan strongman Moammar Gadhafi emerged Sunday with news of a warehouse full of charred corpses and a nanny to Gadhafi's grandchildren tortured.

Meanwhile, rebel forces who have driven Gadhafi and his family into hiding advanced toward his hometown. And CNN has found Abdel Basset al-Megrahi, the man convicted of blowing up a Pan Am jet in 1988, comatose on what his family says is his deathbed.

On the eastern side of Tripoli, troops commanded by Gadhafi's son Khamis killed an estimated 150 captive civilians, as they retreated last week, hurling grenades and spraying bullets into the building full of men they had promised to release, a survivor said.

The massacre took place Monday, the survivor, Muneer Masoud Own, told CNN. Rebels advancing on Tripoli discovered the bodies charred beyond recognition in a warehouse next to the military base. In addition, a nearby resident told CNN that at least 22 bodies were found in a ditch near the base, but it was not clear whether those remains were connected to the killings at the warehouse.

The base is on the main road from Tripoli to the city's airport, which the rebels secured Friday.

Own, 33, said he and his 30-year-old brother were held there for 18 days after being arrested by Gadhafi's forces. He said there were about 70 prisoners held in the warehouse, some of them for up to six months. They ranged in age from a 70-year-old man to a 17-year-old boy.

Monday, with rebel forces pushing into the capital, their guards told them they would be released by sunset, Own said. Instead, he said, just before sunset, the guards opened fire on the men inside, some of whom managed to escape. He has not found his brother.

Gadhafi, who ruled Libya for four decades, and his family are now fugitives, hunted by the rebel forces that toppled his government after a six-month revolt backed by Western airpower. Rebel fighters picked through his family's seaside villas on Sunday, finding high-end stereo equipment, hot tubs and wines valued at hundreds of dollars a bottle, despite an official ban on alcohol.

And they found the horribly scarred Shwygar Mullah, the Ethiopian maid who cared for the children of Gadhafi son Hannibal. Mullah told CNN that Hannibal Gadhafi's wife Aline twice expressed her displeasure with her work by scalding her with boiling water -- then refused to get her medical attention, leaving her scalp and face covered in a mosaic of scars and raw wounds.

Read more about the nanny's experience

Her offense: Failing to beat a daughter who refused to stop crying.

"She took me to a bathroom. She tied my hands behind my back and tied my feet," said Mullah, who worked for the Gadhafis for about a year. "She taped my mouth, and she started pouring the boiling water on my head like this," she said.

Another of the Gadhafi household staff, a man from Bangladesh who did not want to be identified, corroborated Mullah's story and said he also was regularly beaten and slashed with knives.

And in an upscale neighborhood of the capital, convicted Lockerbie bomber Abdel Basset al-Megrahi lay in a coma, CNN Senior International Correspondent Nic Robertson discovered. His family says al-Megrahi is in his last days of a battle with cancer, dimming hopes that he would shed more light on the 1988 attack that killed 270 people.

Read more about al-Megrahi's condition

Robertson found al-Megrahi under the care of his family in his palatial Tripoli villa Sunday, surviving on oxygen and an intravenous drip. His son, Khaled al-Megrahi, said they took him from a hospital as the rebels advanced on the capital and have been treating him at home.

"There is no doctor. There is nobody to ask. We don't have any phone line to call anybody," Khaled al-Megrahi said.

Al-Megrahi was freed from a prison in Scotland in 2009 after serving eight years of a life sentence for blowing up the Pan Am jet, killing all 259 on board and 11 in the town of Lockerbie below. Doctors who had been treating him for prostate cancer gave him just three months to live, and he was released on compassionate grounds.

He received a hero's welcome in Tripoli, enraging many in the United States and Britain. And with the recent overthrow of longtime Libyan strongman Moammar Gadhafi, politicians on both sides of the Atlantic have called for al-Megrahi to be sent back to prison.

But the National Transitional Council, the rebel movement that toppled Gadhafi, announced Sunday that it won't allow the dying al-Megrahi to be extradited.

"We will not give any Libyan citizen to the West," NTC Justice Minister Mohammed al-Alagi said.

The rebels extended their control over the country on Sunday. Spokesman Shamsuddin Ben Ali said NTC forces have taken over the western Libyan city of Ben Jawad, about 100 miles east of Gadhafi's hometown of Sirte.

And a senior rebel military official said about 14,000 rebel fighters were ready to "liberate" Sirte but have given Gadhafi forces there until Monday to disarm and let the rebels enter the city peacefully.

"All the Libyan people are aware of the fact that the people in Sirte are Libyan," NTC military spokesman Ahmed Bani said.

Separately, CNN saw rebels in control of key towns along the road from Tripoli west to Tunisia, giving their commanders hope they can start bringing in supplies to ease a growing crisis in the capital. Alamin Belhaj, a member of the NTC leadership, told CNN that its priorities are now the liberation of Tripoli, security, water and fuel services.

One of Gadhafi's sons, businessman Saadi Gadhafi, made another offer to negotiate with the rebels on Sunday and sounded prepared to cut loose his father and another son once assumed to be his heir.

"If (the rebels) agree to cooperate to save the country together (without my father and Saif), then it will be easy and fast. I promise!" Saadi Gadhafi wrote to Robertson, referring to Moammar and Saif al-Islam Gadhafi.

Saadi Gadhafi said the NTC cannot "build a new country without having us in the table." He predicted the rebels would break into two camps, "Islamists and others," allowing the Gadhafi regime "to choose who we will support so we will come back any way."

CNN's Arwa Damon, Kareem Khadder and Dan Rivers contributed to this report.