Previously undisclosed evidence held by the US intelligence authorities backs up long-held suspicions that a Palestinian militant group, rather than a single Libyan national, masterminded the Lockerbie bombing, according to a new investigation broadcast by Channel 4.

On the 25th anniversary of the bombing of Pan Am flight 103, which led to the deaths of 270 people, a specially commissioned report by Channel 4 News claims that a CIA agent, Dr Richard Fuisz, was given detailed information from within US intelligence and from 15 high-ranking Syrian officials in the immediate years after the December 1988 bombing.

The Syrians revealed to him that they had been in "constant" contact with Ahmed Jabril, the leader of the Popular Front for the Liberation of Palestine - General Command (PFLP-GC), over a five year period shortly after the Lockerbie bombing and regarded the group as being behind the bombing.

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The CIA briefing to Dr Fuisz, in the months after bombing, also claimed the PFLP-GC, then based in Syria, had organised the mid-air destruction of the Pan Am jet.

A Libyan, Abdelbaset al-Megrahi, was convicted in 2001 of mass murder in a unique trial before Scottish judges which saw "neutral" territory in the Netherlands converted to a Scottish court.

Megrahi was jailed for 27 years, but was released on compassionate grounds in 2009 suffering from cancer. He died in Libya in 2012 still pleading he was innocent of the worst terrorist atrocity ever to have taken place on European soil.

Despite detailed investigations over the last 20 years on the forensic evidence and circumstance behind Megrahi's conviction, Scottish and UK authorities have refused to open an inquiry that would re-examine the entire case.

The official explanation remains that it was Megrahi who placed a bomb in a suitcase at Luqa aiport in Malta, which later transferred to Frankfurt and then Heathrow. En route to New York flight 103 blew up at 31,000 ft above Dumfries.

Although the Palestinians and Jabril were the early suspects, the US authorities were instrumental in the switch that subsequently focused on Libya and Megrahi.

Declassified US intelligence documents have previously cited the Iranian government as being motivated to avenge destruction of an Iran Air jet which was shot down by a US battleship over the Persian Gulf in the months before Lockerbie. 290 people on board the aircraft were killed.

In the new account by Channel 4 News, Dr Fuisz is described as giving two US court depositions in late 2000 and 2001 detailing the information he had from Syria.

However this information arrived too late to be used in Megrahi's trial. Three CIA officers (not named) and a US Department of Justice lawyer are alleged to have attended hearings where Dr Fuisz revealed what he knew of the PFLP-GC involvement.

Transcripts of the hearings and other linked documents, never before made public, were uncovered by John Ashton, an investigator who worked with Megrahi's legal team between 2006 and 2009. He has written a new book on the bombing called Scotland's Shame: Why Lockerbie Still Matters.

On the new evidence, Mr Ashton said : "This is yet another indication that the real Lockerbie bombers got away and that Abdelbaset al-Megrahi was wrongly convicted. The British and American governments declared in 1991 that Libya was solely responsible for the bombing, yet for years after senior Syrians were saying that the PFLP-GC was responsible, It seems it was an open secret that the real bombers lay outside Libya."