Mandy Power, 34, her bed-ridden mother Doris Dawson, 80, and her daughters Katie, 10, and Emily, eight, were bludgeoned to death with a heavy iron pole before their South Wales home was set on fire.
Power was beaten so severely that her skull was broken in 10 places, while Emily had 20 skull fractures.
Firefighters found them dead at their home in Clydach, a small village in the Swansea Valley in June 1999.
In 2006, former scrap metal dealer David Morris was jailed for a minimum of 32 years for the murders following a retrial, after an earlier conviction had been ruled unfair by the Court of Appeal.
His conviction brought to an end one of the largest and most complex murder investigations ever undertaken by a Welsh police force.
But last month a BBC Wales Investigates programme questioned the conviction of Morris, who has long maintained his innocence.
It featured interviews with two potential witnesses. One said he had never spoken to police and the other said he contacted police to report what he had seen but nobody ever called him back.
In a statement, South Wales Police said: "The programme featured interviews with two members of the public who claimed they had not previously spoken to police.
"As you would expect, officers have now spoken to both men to establish what information they hold.
"We are carefully assessing this information and will determine our next course of action in due course."
Morris was arrested after the finger of suspicion had wrongly pointed to Power's lover Alison Lewis.
Former policewoman Ms Lewis and her ex-husband Stephen Lewis, an officer with South Wales Police, were arrested on suspicion of murder a year after the deaths.Mr Lewis's brother Stuart, also a police officer, was arrested on suspicion of perverting the course of justice.
They were all released without charge.
Morris’;s conviction has been considered by the Criminal Cases Review Commission more than once, most recently in 2018.
South Wales Police said the matter was not referred to the Court of Appeal “as no new evidence had been identified”.
Additional reporting by PA