The provisional government in Libya says investigators have found a mass grave believed to hold remains of those slain in the 1996 massacre of 1,200 inmates at Tripoli's notorious prison.Libya's new rulers said Sunday that investigators had found the site of a mass grave believed to contain human remains from what many here regard as one of Moammar Kadafi's signature crimes — the 1996 massacre of about 1,200 inmates at Tripoli's notorious Abu Salim prison.