The report, released Thursday, says 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."
Hariri was killed on Feb. 14 in central Beirut in an explosion that killed 17 other people. The Lebanese opposition has blamed Syria and its Lebanese allies, who have both denied any involvement.
The report does not directly assign blame, saying the causes could not be determined.
"However, it is clear that the assassination took place in a political and security context marked by an acute polarization around the Syrian influence in Lebanon," the report said.
President Emile Lahoud urged U.N. Secretary General Kofi Annan late Thursday to do "what is necessary" to uncover the truth behind the Hariri assassination, signaling Lebanon's acceptance of an international inquiry into the Feb. 14 bombing that killed the nation's most prominent politician.
The anti-Syrian Lebanese opposition has accused the government and its Syrian backers of having a hand in the assassination, a charge both the Beirut and Damascus administrations vehemently deny.
The opposition and Hariri's family have insisted on an international investigation, saying they do not trust a Lebanese probe. Some opposition politicians are envisioning an inquiry with powers to investigate and if necessary bring to justice in an international court those indicted in the case.
It also says Syrian Military intelligence bears responsibility to the extent that it and Lebanese security services failed to provide "security, protection, law and order" in Lebanon.
The opposition and Hariri's family have insisted on an international investigation, saying they have no trust in the Lebanese probe. The report implicitly backed that sentiment, saying the Lebanese investigation "lacks the confidence of the population necessary for its results to be accepted."
Hariri's 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 toward the border and has been promising to work out their complete removal with the pro-Syrian government in Beirut.
The investigation was carried out by a team led by deputy Irish Police Commissioner Peter Fitzgerald, appointed by U.N. Secretary-General Kofi Annan at the behest of the U.N. Security Council.
In his report, Fitzgerald also faults Syria for interfering in the governing of Lebanon "in a heavy-handed and inflexible manner."
"Without prejudice to the results of the investigation, it is obvious that this atmosphere provided the backdrop for the assassination of Mr. Hariri," he said.
The report went on to demand an international independent commission with the authority to interrogate witnesses, conduct searches and other tasks. Fitzgerald said such an inquiry would be impossible without Lebanon's cooperation.
Later in the day, Lebanese President Emile Lahoud told Annan to do "what is necessary" to unveil the truth.
The report said the explosion was caused by a TNT charge of about 1,000 kilograms, most likely above the ground.