Police Scotland is examining allegations made by the Justice for Megrahi (JFM) group about the prosecution ofAbdelbaset Ali Mohmed al-Megrahi.
Megrahi, who died in 2012 after being controversially released from prison three years earlier, is the only person to have been convicted over the 1988 bombing of Pan Am Flight 103.
Officers started a probe – Operation Sandwood – in February 2014 after JFM made a number of complaints against prosecutors, police and forensic officials, alleging attempts to pervert the course of justice ahead of Megrahi’s trial at Camp Zeist in the Netherlands 18 years ago.
The completed police report will be passed to independent legal counsel before it goes to the Crown Office.
Iain McKie, a member of JFM, said he expected the police report to be concluded in the next few months.
He said: “The more information Police Scotland got during the course of the inquiry, the more long and involved the investigation became. All credit to them, they have followed where that took them.
“It would appear as if the report is very near to completion. I can well understand why it’s late. As an ex-cop myself, I know that some inquiries can grow arms and legs.”
Last year Megrahi’s family lodged a new appeal against his conviction with the Scottish Criminal Cases Review Commission (SCCRC).
It is believed the appeal is based on concerns over the evidence used to convict the Libyan, including that given by Maltese shopkeeper Tony Gauci, who died in 2016.
Mr McKie added: “[Operation Sandwood] is going to be the definitive inquiry because it’s going to be used by the Scottish Criminal Cases Review Commission in looking at the current appeal from the Megrahi family and also to underpin our call for a public inquiry.
“This is an extremely important report – it’s probably the last time the people of Scotland will get the chance to look behind the muck that covers Lockerbie.
“There’s no pressure from us about the time it takes. It’s the quality of the report we’re interested in.”
It is not the first time hopes have been raised about the long-running police investigation coming to an end. In 2015, police said the inquiry was in its “final stages”.
Last year former First Minister Alex Salmond used his show on Kremlin-backed broadcaster RT to claim Megrahi may have been “wrongly convicted”.
Mr Salmond, who was first minister at the time of Megrahi’s release, said the evidence used to convict him was “open to question”.
The former SNP leader said he believed it was possible for someone to be guilty, but also wrongly convicted.
Detective Superintendent Stuart Johnstone said: “The investigation has reached its concluding phase and the full report will follow with a submission process through the deputy chief constable to the independent QC appointed by Police Scotland, prior to submission to the Crown.”