The Scottish Liberal Democrat leader has written to the Lord Advocate to seek assurances that the Lockerbie bombing is still being "rigorously and actively investigated".
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.
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 said 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 Mr 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."
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."
A Crown Office spokesman said: "The Lord Advocate has twice this year met the interim Libyan Prime Minister and has sent a detailed International Letter of Request to the Libyan Government. At the second meeting the interim Libyan Prime Minister asked for detailed information about the nature of the investigation and this has been provided in two letters.
"The Lord Advocate awaits a response and the Scottish Police await visas from the Libyan Government in order to carry out their investigation in Libya. Crown Office officials are also in regular contact with the Foreign and Commonwealth Office in relation to the ongoing investigation."