LONDON (Reuters) - No evidence has emerged that casts doubt on the conviction of a late Libyan intelligence officer for the Lockerbie airline bombing 26 years ago, says Scotland's top prosecutor.

However, efforts to track down others behind the deadly attack have been hampered by the violence that has rocked Libya for months, Lord Advocate Frank Mulholland will tell a memorial service on Sunday, according to an advance copy of his speech.

Abdel Basset al-Megrahi was the only person found guilty over the Dec. 21, 1988, attack on Pan Am Flight 103 in which 270 people were killed. His family and some relatives of the Scottish victims believe he was wrongly convicted.

He was jailed for life in 2001, but was released by Scotland's government on compassionate grounds eight years later after being diagnosed with terminal cancer. Megrahi, who always protested his innocence, died in Libya in 2012.

At the memorial service for Lockerbie victims in Washington, Mulholland, the head of Scotland's Crown Office, will say nothing had come to light to question Megrahi's guilt.

"During the 26-year long inquiry not one Crown Office investigator or prosecutor has raised a concern about the evidence in this case," he will say.

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

Most of the victims of the explosion over the Scottish town of Lockerbie in Scotland were Americans on their way home for Christmas. Eleven people died on the ground as the New York-bound jet crashed when a bomb exploded in its hold some 40 minutes after leaving London's Heathrow airport.

Mulholland will reaffirm a commitment made by the United States, Britain and Libya a year ago on the 25th anniversary of the attack to hunt down all those responsible.

"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," he will say. "The Crown will never give up the fight to secure justice for the families of those who died."

In 2003, former Libya leader Moammar Gadhafi accepted his country's responsibility for the bombing and paid compensation to the victims' families, but did not admit personally ordering the attack.

However, some continue to dispute Megrahi's guilt.

"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?" Jim Swire, whose daughter Flora died in the bombing, told the BBC.

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