Scotland's chief law officer believes there is a "realistic possibility" of a second trial over the murder of 270 people in the Lockerbie bombing.

Scottish and American investigators announced last year that they had identified two Libyans as suspects over the 1988 atrocity but since then very little has been said publicly about the case.

In an interview with STV News to mark his departure from the post after five years, lord advocate Frank Mulholland QC discussed the prospect of fresh prosecutions over Britain's biggest mass murder.

"I've been to Tripoli twice," said. "I've established good relations with the law enforcement attorney general in Libya.

"We're currently at a stage where there are a number of outstanding international letters of request, one of which is seeking the permission of the Libyan authorities to interview two named individuals as suspects.

"Following all the work that's been going on, and it's been painstaking, it's taken some time, it does take time.

"I hope that the Libyans will grant permission for that to be done. I obviously can't say too much publicly but a lot of work is going on behind the scenes to make that happen.

"What I hope is that this will bear fruit and we can take it to the next stage of seeking the extradition of the two named individuals."

Last October, it was announced the lord advocate and the US attorney general had agreed there was "a proper basis in law" to treat the two Libyans as suspects.

The two men were not named by the Scottish or US authorities but they are Abdullah Senussi, Colonel Gaddafi's former intelligence chief, and Abouajela Masud.

Both are being held in jails in Libya - Senussi is appealing against a death sentence while Masud is serving ten years for bomb making.

Libya has been plagued by violence since the 2011 uprising against Gaddafi, with IS gaining a foothold in the country.

Foreign secretary Philip Hammond travelled to Tripoli last month in support of the new UN-backed government of national accord, a visit welcomed by the lord advocate.

Asked if there was any realistic possibility of Senussi being surrendered for trial, Frank Mulholland replied: "Before I embarked on this work I was told that there was no possibility, absolutely none, of the Libyans cooperating with law enforcement in Scotland or the United States. That happened.

"In 2011, I attended a ceremony in Arlington where the Libyan ambassador to the US made a public commitment on behalf of the Libyan government to help. They have kept their word. They have helped.

"I said it takes time, and it will take time, and that's certainly something which we are used to in relation to the Lockerbie inquiry.

"If we get to the stage of seeking the extradition of two named individuals or indeed more persons, I think there's a realistic possibility that there could be a further trial."

The two men are suspected of bringing down Pan Am 103 while acting along with Abdelbaset al-Megrahi, who remains the only person convicted of the bombing.

He died protesting his innocence after being released on compassionate grounds by the Scottish Government. A high-profile campaign to clear his name continues.

The lord advocate acknowledged any new Lockerbie trial would involve a public re-examination of the disputed evidence from Megrahi's.

"I don't fear that," he said. "I think that's a good thing. Without seeking to comment on what the outcome would be, I think the evidence would stand up to a further test.

"We wouldn't be doing this unless we thought that the evidence was sufficiently credible and reliable to have them interviewed as suspects, I think that's the best way to put it."

For many years after the bombing it seemed extremely unlikely there would ever be prosecutions over Lockerbie.

Eventually a diplomatic deal paved the way for the first trial to go ahead in a specially-convened Scottish court sitting at Camp Zeist in the Netherlands.

Frank Mulholland first raised the hope that the collapse of Gaddafi's regime could allow Scottish police to visit Libya back in 2011.

He is the first British or American official to publicly express the belief that a second trial could happen, albeit with carefully chosen words.