A mass grave believed to contain up to 1,270 bodies has been found in the Libyan capital, Tripoli, says the National Transitional Council (NTC).The remains are thought to be those of inmates who were killed by security forces in 1996 in the Abu Salim prison.The uprising against Col Muammar Gaddafi began as a protest to demand the release of a lawyer who represented families of the Abu Salim inmates.Excavation at the site is expected to start soon.The NTC said it had discovered the site - a desert field scattered with bone fragments within the grounds of the Abu Salim prison - by questioning prison guards who had worked there when the prisoners were killed after protesting against their conditions.Several bone fragments and pieces of clothing have already been found in the top soil.'Grenades and gunfire'Some family members visited the site, among them Sami Assadi, who lost two brothers in the incident.He was told they had died of natural causes only five years ago. He told the BBC how it felt to be at the place where his brothers may be buried."Mixed feelings really. We are all happy because this revolution has succeeded, but when I stand here, I remember my brothers and many, many friends have been killed, just because they did not like Muammar Gaddafi."Until recently, little was known about the circumstances in which the prisoners died, says the BBC's Jonathan Head who went down to the site.A few eyewitnesses have talked about the fact they were killed in their jail cells by grenades and sustained gunfire after a protest.Officials in the new government say they will need foreign forensic help to determine exactly what happened there.