Syria denies involvement in his killing, which took place after he accused Damascus of interfering in Lebanese politics.
Street protests in Lebanon in the wake of al-Hariri's killing prompted Syria to withdraw forces that had been in the country for 22 years.
The Lebanese government and the Security Council approved plans to establish a special tribunal to try al-Hariri's killers, but it has not yet been ratified by the Lebanese parliament.
Nabih Berri, Lebanon's parliamentary speaker and a key member of the Hezbollah-led opposition to the current government, has not said when he will convene parliament.
Brammertz said when his inquiry was complete, it would be handed over to a prosecutor, who would finalise the investigation and prepare indictments.
He is also helping Lebanese authorities investigate 16 other politically motivated cases, including last November's assassination of Pierre Gemayel, Lebanon's industry minister.