Media playback is unsupported on your device Media caption Mulholland: "No-one has raised any concerns about the evidence to me"

Scotland's top prosecutor has reaffirmed his belief that Abdelbaset al-Megrahi is guilty of killing 270 people in the Lockerbie bombing.

Lord Advocate Frank Mulholland said no Crown Office investigator or prosecutor had ever raised concerns about the evidence used to convict Megrahi.

He also pledged to continue tracking down Megrahi's accomplices.

Megrahi's part in the bombing has been called into question in a series of books and documentaries.

And a petition seeking "Justice For Megrahi", backed by politicians and family members of some victims, remains on the Scottish Parliament's books two years after his death.

Image copyright PA Image caption Megrahi was convicted of mass murder in 2001

Supporters of Megrahi, who was said by the FBI to have been a Libyan intelligence officer, have claimed that Scottish prosecutors ignored evidence that the bomb was put on board the flight at Heathrow rather than in Malta.

They have also alleged Libya was "framed" over the bombing, and that a fragment of the bomb's timer was either planted or manipulated to implicate the North African country and to turn attention away from Syria and Iran.

Speaking as he travelled to Washington to attend a memorial service at Arlington cemetery in Washington on Sunday, Mr Mulholland said his ongoing investigation into the bombing was focused on the evidence, and not on "speculation and supposition".

Mr Mulholland said: "During the 26-year long inquiry not one Crown Office investigator or prosecutor has raised a concern about the evidence in this case.

"We remain committed to this investigation and our focus remains on the evidence, and not on speculation and supposition.

"Our prosecutors and police officers, working with UK government and US colleagues, will continue to pursue this investigation, with the sole aim of bringing to justice those who acted along with al-Megrahi."

Mr Mulholland later told the BBC that the only place to determine guilt or innocence was in a court of law, where the evidence could be subjected to "great scrutiny, cross examination and testing".

Image copyright Reuters Image caption 270 people died when Pan Am flight 103 exploded over Lockerbie

Image copyright Reuters Image caption Megrahi continued to protest his innocence until his death in 2012

The Lord Advocate added: "We had one trial, two appeals, 13 judges involved, and at the end of that process Megrahi remains a convicted person.

"Also, and highly significant in my view, Megrahi abandoned his second appeal knowing that the effect of that abandonment was that he would remain guilty of the Lockerbie bombing."

But Jim Swire, whose daughter Flora died in the bombing, said he continued to believe that Megrahi was innocent.

He said: "How was it that the Scottish Criminal Cases Review Committee spent three years looking at the case and came up with six pieces of evidence to challenge Mr Megrahi's conviction?

"I think you have to look further than the superficial comments made by the Lord Advocate."

Media playback is unsupported on your device Media caption Dr Jim Swire: ''We don't feel we've been told anything like the truth''

He said he believed that evidence was "clearly designed in order to mislead the court" and pointed the finger towards the CIA.

Mr Swire said recent revelations about the agency's complicity in torture following the 9/11 terrorist attacks showed the CIA "has little respect for the requirements of the law of their own country."

Mr Swire also repeated his call for the evidence in the case to be examined "properly and objectively" in a full inquiry - something he said had been turned down by every prime minister since the bombing "for whatever reason".

'Unquestioned integrity'

Former Liberal Democrat leader Sir Menzies Campbell criticised as "unsavoury" the "repeated and unfounded criticism" of the judges who sat in the trial, and those who heard Megrahi's subsequent appeal against conviction.

Sir Menzies added: "The judges by convention are unable to respond publicly to these criticisms, which imply that they were somehow part of a conspiracy. In truth, they were all experienced in criminal law and of unquestioned integrity.

"It is a curious feature of this case that those who argued most vehemently for a special court to be set up to deal with the case are now among the most vociferous critics of its verdict."

Image copyright Getty Images Image caption Megrahi returned to Libya after controversially being released on compassionate grounds by the Scottish government in 2009

Megrahi was convicted of carrying out the bombing in January 2001, but co-accused Al Amin Khalifa Fhimah was acquitted.

Fhimah was Libyan Arab Airline's station manager at Luqa Airport in Malta, where the two men were alleged to have loaded the bomb aboard an Air Malta flight to Frankfurt before it was transferred to a feeder flight for Pan Am 103.

There has been newspaper speculation that prosecutors continue to believe Fhimah was involved in the plot.

'Wider investigation'

But a Crown Office spokeswoman told BBC Scotland: "Fhimah is not the focus of the ongoing investigation. However, if any further evidence against him comes to light it will be considered as part of the wider investigation."

The Pan Am flight exploded at 31,000ft over Lockerbie, in the south of Scotland, on 21 December 1988.

As well as 259 people on board the aircraft, 11 residents of Lockerbie died on the ground as a result of a giant fireball caused when a wing holding thousands of gallons of fuel exploded on impact.

The Scottish government released Megrahi on compassionate grounds in August 2009 after he was diagnosed with terminal cancer and given six months to live.

He returned to Libya, where he died in May 2012, still protesting his innocence.