SCOTLAND'S most senior law officer called for a "redoubling of efforts" to bring all those involved in the Lockerbie bombing to justice, as he confirmed detectives were expected to head to Libya early next year.

Speaking from Washington yesterday, Lord Advocate Frank Mulholland said he would be "failing in his duty" if he did not do all in his power to find all those who were responsible for the bombing, adding the job had "only been partly done".

He said he hoped officers would be in Libya "in the new year, as early as possible" and it was "very important" to get some movement now.

His comments came on the 23rd anniversary of the Pan Am Flight 103 atrocity, in which 270 people died, and the day after he met FBI director Robert Mueller and US Attorney General Eric Holder in Washington.

The meeting with Mr Mueller and Mr Holder was with a view to stepping up the investigation.

Mr Mulholland said: "What we have is a window of opportunity with the new Libyan Government and we should seek to exploit that and redouble our efforts. This was the reason why I met with the attorney general and the director of the FBI.

"I was very pleased they completely agreed that law enforcement agencies have to redouble their efforts and take advantage of this great opportunity that became evident following the overthrow of [former Libyan leader Colonel] Gaddafi."

It marks a turnaround in co-operation between Scotland and the US after relations hit rock bottom when Abdelbaset Ali Mohmed al Megrahi, 59, who remains the only person to be convicted and sentenced in connection with the bombing, was controversially freed from a Scottish prison on compassionate grounds in August 2009.

Megrahi was dying from prostate cancer and doctors advised he had around three months to live when he was released.

Some doubt the official version of events which put the blame solely on Libya. Alternative theories state it was the work of Iranian intelligence, or a Palestinian terrorist group.

It is believed the Lockerbie investigators, led by officers from Dumfries and Galloway Constabulary, want to speak to Abudullah Senussi, the intelligence chief and brother-in-law of Gaddafi. Known as "The Executioner", Senussi was reportedly captured helping Gaddafi's son, Saif, last month.

Mr Mulholland, who was promoted to the role of Scotland's chief prosecutor after May's elections, set up a double jeopardy unit to look at failed prosecutions and has focused on Lockerbie.

His statement in the US is seen as his toughest yet in committing to seeking justice for the Lockerbie families.

Mr Mulholland said the investigation would look to find out "who is available, who is alive, where they are and what the evidence is against them".

"Steps are in place to gain access [to Libya] through normal international protocols, setting out what evidence you are particularly interested in and we hope to have that through the Foreign Office with the new Libyan Government as quickly as possible," he said.

"The trial court held this was an act of state-sponsored terrorism. Megrahi himself worked for the Libyan security service and I am sure there is evidence in Libya which will give the families the answers they seek as to the involvement of others.

"I welcome the very positive statements that have been made by the new Libyan Government, seeking to co-operate with the Scottish and US law enforcement agencies and allowing them access to Libya to seek the evidence, the answers, to bring the others involved to justice."

Libya's Ambassador to the US, Ali Aujali, was to address relatives of the victims at yesterday's remembrance service for the victims of the Lockerbie bombing at Arlington National Cemetery in Washington.

It is thought to be the first time a Libyan politician has attended an event to commemorate Lockerbie.

Mr Mulholland said it was "very symbolic".

He said: "Libyan Government ministers have been making statements that are very welcome.

"It is very symbolic the new Libyan ambassador is attending. I think that is a huge symbolic statement and it is very welcome."

The Lord Advocate was expected to lay a wreath on behalf of the people of Scotland and made a short speech in remembrance of the victims of the atrocity. He was later to meet the US relatives of the victims.

He said it was "a terrible loss of life", adding "they have my support and my sympathy".