A team headed by Deputy Commissioner Peter Fitzgerald was sent to Lebanon a few weeks after the fatal explosion and charged with investigating the initial inquiry.
The report was scathing and demanded a fresh international investigation after concluding Lebanese authorities messed up or even manipulated the probe.
Syria and its allies within Lebanon were not then blamed for ordering and planning the bombing that killed the ex-PM and 20 other people in February.
Mr Fitzgerald carried out the probe with the help of two senior members, Detective Chief Superintendent Martin Donnellan of the National Bureau of Criminal Investigation and Supt Pat Leahy of the Operational Support Units.
Underlying political tensions, fermented by Syria, were identified by the team as a prime reason for the assassination.
“Clearly, Mr Hariri’s assassination took place on the backdrop of his power struggle with Syria, regardless of who carried out the assassination and with what aim,” the report said.
The team took testimony that Syrian President Bashar Assad had threatened Mr Hariri and opposition figure Walid Jumblatt with physical harm.
Syria’s Ambassador to the United Nations Fayssal Mekdad dismissed the report. It contained “too much rhetoric”, it said.
The toughest criticisms were reserved for the Lebanese authorities charged with investigating the murders. There was a “distinct lack of commitment” and the probe was not carried out “in accordance with acceptable international standards”.
Among the flaws identified were the disappearance of crucial evidence and tampering with the scene of the blast. Parts of a pick-up truck were brought to the scene, placed in the crater and photographed as evidence, it said.
The report alleged investigative judges had no control over the probe and even faulted police for not turning off a water main that flooded the blast crater and washed away vital evidence.
Serious doubts were raised about suspects in the bombing, a Palestinian named Ahmed Abu Adas, and a group that claimed responsibility, the little-known Support and Jihad in Syria and Lebanon.