The double agent who may solve Lockerbie's final puzzle

Lockerbie trial: Abdul Majid Giaka, known to the CIA as Puzzle Piece, claims he will reveal who bombed Pan Am flight 103

The voice was warped and metallic, its source hidden behind a glass screen 10ft high. On the 50th day of the Lockerbie trial the prosecution unveiled its star witness, Puzzle Piece WK140, with the claim that the secrets of the destruction of Pan Am flight 103 will at last be revealed.

The voice was warped and metallic, its source hidden behind a glass screen 10ft high. On the 50th day of the Lockerbie trial the prosecution unveiled its star witness, Puzzle Piece WK140, with the claim that the secrets of the destruction of Pan Am flight 103 will at last be revealed.

The testimony of Abdul Majid Giaka (Puzzle Piece in CIA code) will either make the case against the two Libyans charged with the bombing or break it with the shattering international political and diplomatic repercussions that that will entail.

Mr Giaka, a Libyan agent who had been working for the Americans, is the only person who links the two defendants, Al Amin Khalifah Fhimah and Abdulbaset Ali Mohmed Al Megrahi, and the Libyan secret service with the bomb, which blew up the New York-bound jumbo over Lockerbie on 21 December 1988, killing all 259 on board and 11 on the ground.

Mr Giaka claims to have seen the two men with the Samsonite suitcase of the type used in the blast and also discussed placing an unaccompanied bag containing a bomb on board a plane with Mr Al Megrahi.

The double agent's evidence at the Scottish Court sitting at Camp Zeist in the Netherlands comes after a succession of prosecution witnesses have been badly mauled by defence questioning. There is now a tacit acceptance by both sides that this is the most crucial part of the trial. If Mr Giaka's testimony can be discredited the case will almost certainly collapse.

Mr Giaka is one of the most heavily protected "intelligence assets" in the western world. He could also become one of the wealthiest, in line for a $4m (£2.7m) reward offered by the FBI to anyone securing the conviction of the bombers. His appearance yesterday followed a month of legal wrangling over his security arrangements and how much classified information the CIA should hand over to the defence.

Mr Giaka, who claims to be in daily fear of assassination by Colonel Muammar Gaddafi's hitmen, had given his statement to the prosecution while travelling in a bus wearing a Shirley Bassey wig. At his appearance yesterday he had insisted not only on disguise, although whether he had remained loyal to Ms Bassey in his choice of headwear remained unclear, but also a triple reinforcedbullet-proof screen and electronic distortion of his voice.

Mr Giaka was accompanied by no fewer than 30 armed US marshals who guard him around the clock, and the complex was surrounded by armed British and Dutch police.

Mr Giaka began his testimony at 9.50am from his secure vantage point just eight feet away from where his two former colleagues, Mr Fhimah and Mr Al Megrahi, sat in the dock between Scottish police and prison officers. Both wore shashiya caps on their heads and dark waistcoats. They sat forward, intently listening to every word coming from behind the screen. The two men deny conspiracy to murder, murder and a breach of the 1992 Aviation Security Act.

The witness, defendants and array of lawyers were separated by another glass screen from the rest of the court, where relations of the victims and the accused sat, with the media and more security staff. Occasionally a small group of Libyans, including two women from the defendants' families, and relations of the bereaved would look at each other and then quickly look away.

Dr Jim Swire, whose daughter Flora was one of the dead and who has campaigned ceaselessly to find out what happened, sat in the front row of the seats just a foot away from where Mr Giaka was giving evidence. He stared straight ahead at the opaque screen in front, willing himself to look at the face of the man who claims to know how and why his daughter was killed.

With his answers to questions being translated into English and Arabic, Mr Giaka described how he was recruited into the Jamarhiya Security Organisation (JSO), the Libyan secret service, and then decided to defect to the Americans.

He said: "I felt uncomfortable because of the JSO's involvement in terrorism and because it was involved in the assassination of dissidents outside Libya. I contacted the American embassy because I wanted to leave my post and go to the United States. But it became apparent that there was a big opportunity to get information [for the CIA] about Libyan intelligence and its movements."

The CIA paid him $1,000 a month and arranged for him to have medical operations to "aggravate" an old injury so he could avoid conscription into the Libyan army.

Mr Giaka worked with Mr Fhimah and Mr Al Megrahi at the offices of the Libyan Arab Airlines in Malta. In the summer of 1986 a Libyan agent could Said Rashid came to Malta and spoke to Mr Giaka to investigate the possibility of placing an unaccompanied bag on board a British aircraft, the witness told the court.

Mr Giaka claimed that he discussed the matter with Mr Al Megrahi and told him not to "rush things". He said he saw Mr Fhimah once take a Samsonite suitcase from the luggage carousel at Malta airport and leave without the case being inspected by Customs.

Mr Giaka told the court that Mr Fhimah once unlocked a desk drawer at the airport to show him two bricks of yellow-coloured explosives packed into paper boxes and covered up with baggage labels.

"Mr Fhimah told me he had kilos of TNT which were delivered to him by Mr Al Megrahi," said Mr Giaka. "The boxes contained a yellowish crystalline substance and I was told they must be kept a secret."

Mr Giaka claimed that he was subsequently asked to take care of the explosives and transferred them to the Libyan consulate in Malta.

Outside court Dr Swire, who has attended almost every day of the trial, shook his head and said: "By his own admission this man is a draft dodger and a traitor to his country. We also know that intelligence agents of every country are taught to lie. But we must listen to what he has got to say and weigh up the truth or otherwise of that. We cannot say he has been lying on this occasion whatever we think of this man's character.

"This is now the most crucial part of the trial and this is the most crucial witness. Whether or not we finally find out what exactly happened to that flight and why so many people were killed will depend very much on this man."

The trial continues today.