Brammertz said the UN inquiry had identified people who may have been involved, though he did not name anyone.
He said investigators had "significantly narrowed down" possible motives for the slaying to al-Hariri's political and personal relationships in Lebanon, Syria and other countries.
Germany's Detlev Mehlis, the first UN chief investigator, had said the plot's complexity suggested that Syrian and Lebanese intelligence services had a role.
Four pro-Syrian Lebanese generals have been under arrest for alleged involvement in the murder.
International pressure and huge protests in Lebanon following al-Hariri's assassination prompted Syria to pull its troops out of Lebanon after a 29-year presence. Syria has denied involvement in the murder.
Bellemare was Canada's assistant deputy attorney general from 1993 until December 2006, overseeing federal prosecutions in the country.
He later became special adviser to Canada's deputy minister of justice until he retired from public service in September, according to a resume attached to Ban's letter.