The Scottish Liberal Democrat leader has sought assurances that the Lockerbie bombing is still being "rigorously and actively investigated".
Willie Rennie said in a letter to the Lord Advocate that the relatives of the victims of the Lockerbie bombing need assurances that the live investigation is still being fully resourced and supported in its efforts to bring any others involved in the bombing to justice.
In his letter to Lord Frank Mulholland he wrote: "In May this year you said: 'The investigation into the Lockerbie bombing will continue, to bring to justice the others involved in this act of state-sponsored terrorism.'
"I hope this is still the case and you can provide assurances that the Lockerbie bombing is still being rigorously and actively investigated."
Abdelbaset al-Megrahi proclaimed his innocence up until his death earlier this year
Former Libyan intelligence officer Abdelbaset al-Megrahi was sentenced to life in prison for the 1988 bombing of a US airliner over the Dumfries and Galloway town which claimed 270 lives.
He was later diagnosed with terminal prostate cancer and controversially released from prison in August 2009, with an estimated three months to live, on compassionate grounds.
But Megrahi, who always proclaimed his innocence, proved medical experts wrong and finally lost his cancer battle in Tripoli in May this year.
The Lord Advocate Frank Mulholland met the interim Libyan prime minister after Megrahi's death to discuss the investigation of others involved in the Lockerbie bombing.
However Liberal Democrat leader Willie Rennie said investigating officers from Dumfries and Galloway Constabulary had not yet been to Libya to follow up on enquiries.
Mr Rennie had previously backed calls in the Scottish Parliament for an inquiry into the Lockerbie prosecution.
He said: "Earlier this year the First Minister rejected calls for an independent inquiry into the Lockerbie prosecution, which could have shone a light on the extent of Megrahi's involvement or even pointed towards others being involved.
"In denying that independent inquiry the opportunity to get the answers so deserved by relatives of the Lockerbie bombing it was left to the criminal investigation.
"It's therefore hugely disappointing that Scottish police officers have yet to visit Libya to undertake their investigation.
"With the passing of each month it becomes less and less likely that any existing evidence can be found.
"The Crown's position that Megrahi did not act alone is all very well, but it brings neither justice nor answers to those who need it most."
Earlier this month Megrahi's biographer described the cancer which killed him as a "gift from God" to establishments with something to hide.
John Ashton made the claim at the Edinburgh International Book Festival where he appeared with Jim Swire, who lost his daughter in the 1988 bombing, and Hans Kochler, the UN observer at the subsequent trial in the Netherlands.
Mr Ashton, who recently published a book on the former Libyan intelligence officer, said: "Megrahi's cancer was a gift from God for everybody involved that had something to hide.
"It allowed his release, it allowed the final stages of the rapprochement between the UK and Libya, and it allowed the Scottish government to allow him out of prison on a legal basis that wasn't one laid down by the hated Government in Westminster.
"It was a tragedy for Megrahi but I think everybody else was punching the air."
But he denied the trial was a "grand conspiracy" involving a range of security services and leading all the way to heads of state such as the US president.
"What I say is, first and foremost, that the judges got it wrong, for whatever reason, and the Crown Office withheld evidence," he said.
"I'm sure they did so in good faith but their behaviour was utterly incompetent and shameful."
Mr Ashton's comments underlined the gulf between those who believe in Megrahi's guilt and those who feel he was innocent or the victim of a miscarriage of justice.
American relatives in particular were angered by Scottish Justice Secretary Kenny MacAskill's decision to free Megrahi under compassionate release rules.