Five police officers who had direct knowledge of some of the earlier murders were among those now willing to speak out against the Ampatuans, according to de Lima, adding that "given the right opportunity there will be witnesses who can exactly pinpoint these mass graves".
Andal Ampatuan Sr, the patriarch of the clan, had been governor of Maguindanao and an ally of the president, Gloria Arroyo.
Police allege his son, Andal Ampatuan Jr, a local mayor, led last month's massacre to stop the challenge of a political rival in next year's elections.
The government allowed the Ampatuans to run a private army as part of a strategy to contain a long-running Muslim separatist insurgency in Maguindanao and other parts of the south.
But the brazenness of the massacre sparked public outrage across the country and Arroyo pledged swift justice against the culprits.
She declared martial law in Maguindanao last week and accused the Ampatuans of rebellion; government forces arrested 62 people, among them Ampatuan Sr and other clan leaders.
Agnes Devanadera, the justice secretary, said Ampatuan Sr would be charged with rebellion and possibly murder.
In a separate press briefing on Wednesday, the nation's police chief said at least 161 people, including police and army officers, were suspected of having directly participated in the November massacre.
"We have so far identified 161 suspects who directly participated in the gruesome massacre," Jesus Verzosa, the national police director general, said.
He cited witnesses as saying that Ampatuan Jr, who has been charged with 25 counts of murder so far, led the group of militiamen who stopped his rival's convoy and shot some of the victims himself.
Giving the most detailed official account of the massacre yet, other police officers told the nationally televised briefing that 26 of the victims were women and 32 were journalists.
Some of the victims' bodies had been mutilated with knives as well as shot in the mouth and chest at close range, police said.