The UN report, submitted on Thursday to the Security Council, cited both Ahmad Abdel-Al - a member of a pro-Syrian Islamic militant group - and his brother Mahmoud Abdel-Al, for their suspected involvement.
The February 14 assassination transformed Lebanon's political landscape and prompted Syria to end its military presence there after three decades. Ahmad Abdel-Al, of the Ahbash group, was a key figure in the plot, the report found.
His brother called the mobile phone of pro-Syrian President Emile Lahoud shortly before the blast that killed Hariri and 22 others near Beirut's seafront, the report added.