Led by Irish Deputy Police Commissioner Peter Fitzgerald, the fact-finding team has now called for an international independent commission to "find the truth".
The report, delivered to Secretary-General Kofi Annan by Fitzgerald, said Syrian military intelligence bore primary responsibility for the lack of security, protection and law and order in Lebanon and said Lebanese security teams showed "systematic negligence" in performing their duties.
The report said there was a "distinct lack of commitment" by Lebanese authorities to investigate the crime, and the investigation was not carried out "in accordance with acceptable international standards".
The report also said it was doubtful that a proper investigation could be carried out with the current Lebanese security apparatus in office.
In his report, Fitzgerald demanded an "international independent commission" with the authority to interrogate witnesses, conduct searches and other tasks.
He said such an inquiry would be impossible without Lebanon's cooperation.
Al-Hariri's killing prompted
several demonstrations in Beirut
Al-Hariri was killed on 14 February in central Beirut in an explosion that killed 17 other people.
The UN fact-finding report said the explosion was caused by a TNT charge of about 1000 kilograms, most likely above the ground.
The killing led to turmoil in Lebanon. Mass demonstrations forced the resignation of the Lebanese government and intensified the international campaign for Syria to withdraw its troops from the country.
Syria has now pulled back its troops and intelligence agents into eastern Lebanon towards the border and has been promising to work out their complete removal with the pro-Syrian government in Beirut.
Meanwhile, in the first official Lebanese reaction to the findings of the investigation, Lebanon's Syrian-backed President Emile Lahud called on Annan on "to do what's necessary" to resolve the questions left unanswered by al-Hariri's assassination.
"Lahoud asked the secretary-general to do what's necessary to reveal the truth in the crime as soon as possible," the Lebanese presidential palace said in a statement.
It said Annan had called Lahud to inform him of the findings of 14 February killing.
In related news, Syria's UN ambassador Faisal Miqdad said the report proves his country had no hand in al-Hariri's assassination.
"We have no ambitions in Lebanon, we simply seek its security and good brotherly relations," he told Aljazeera.
The UN Security Council may now take up the issue, possibly by laying the groundwork for a new inquiry.
The Lebanese opposition welcomed the UN fact-finding report.
"It conforms totally with the political vision of the Lebanese opposition," Ghattas Khuri, a lawmaker from al-Hariri's parliamentary bloc, said.
"We have said there was negligence by the Lebanese security apparatus to the point of collusion and the report came to say that this negligence was organised, which could reach the point of collusion."