SCOTLAND'S top prosecutor has reiterated his belief that Abdelbaset al Megrahi was guilty of killing 270 people in the Lockerbie bombing.

Supporters of the Libyan, who died in 2012, still protesting his innocence, have repeatedly questioned his part in the bombing of Pan Am flight 103, claiming the case against him was flawed and evidence was manipulated.

But the Lord Advocate Frank ­Mulholland, who will attend a memorial service in the US today to mark the anniversary of the disaster, said no Crown Office investigator or prosecutor has raised a concern about the evidence in the case during 26 years of investigation.

Mulholland also pledged to continue the "fight to secure justice" for the families of those who died by continuing to track down Megrahi's accomplices.

He said the investigation would remain focused on evidence and not "speculation and supposition".

Speaking ahead of the ­memorial service, which will take place at Arlington cemetery in ­Washington, Mulholland said: "The current ­instability in Libya has meant that some investigative ­opportunities have required to be reassessed, which I know has been frustrating for family members.

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

"There are other significant ­investigative opportunities open to us which are not reliant on obtaining evidence from our Libyan colleagues.

"The Crown will never give up the fight to secure justice for the families of those who died. It might be 26 years since 270 people lost their lives in the terrorist attack, but justice has no sell-by date in Scotland.

"Over the years many people have worked on the inquiry and all have been given the same instruction: to carefully review the evidence and work to identify all of those who were involved in the conspiracy to destroy Pan Am flight 103."

He added: "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."

Megrahi was convicted of the ­Lockerbie bombing in 2001 and sentenced to life in prison, with a minimum of 27 years.

He was released from prison in Scotland in 2009 by the Scottish Government on compassionate grounds as he was suffering from cancer, a decision that provoked much controversy, particularly when he outlived a survival prognosis of just three months.

Key evidence that led investigators to Megrahi included a timer fragment found in scraps of clothing wrapped around the bomb that detonated on board the flight.

The timer was linked to a batch sold to Libya and Megrahi was ­ identified as the purchaser of the clothes by Malta shop owner Tony Gauci.

But supporters of Megrahi have claimed the timer fragment was either planted or altered to implicate Libya and divert suspicion from states such as Iran.

They have also pointed to a lack of direct evidence to show the bomb was loaded on the plane in Malta, ­claiming instead it could have been loaded at Heathrow.

Dr Jim Swire, whose 23-year-old daughter Flora died in the bombing, has been one of Megrahi's most vocal supporters.

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."

But former Liberal Democrat leader Sir Menzies Campbell said: "One of the most unsavoury elements of this case is the repeated and unfounded criticism of the judges who sat in the trial, and those who heard the subsequent appeal against conviction.

"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."