The investigators, accompanied by Lebanese police, on Thursday searched the home of Colonel Ghassan Tufayli who was head of the Lebanese military intelligence's surveillance unit, the sources said.
There was no comment from the Lebanese army or the UN investigating commission on the report.
Tufayli was allowed to go after several hours of questioning. It was not immediately clear whether he faced possible charges.
Lebanon has already charged four pro-Syrian security generals, including the ex-military intelligence chief, Raymond Azar, in connection with the 14 February killing of al-Hariri and 22 others near Beirut's seafront.
An interim report by chief UN investigator Detlev Mehlis in October said Tufayli's unit had al-Hariri "under permanent wiretapping" and had passed on details of his conversations to Azar and to army chief General Michel Suleiman.
"According to Colonel Tufayli's statement, General Raymond Azar sent the protocols to the Lebanese President (Emile Lahoud) and to General (Rustom) Ghazali, the head of the Syrian Military Intelligence in Lebanon," the report said.
Mehlis implicated senior Syrian and Lebanese officials, including Syrian President Bashar al-Assad's brother-in-law, in the killing.
His findings prompted a unanimous UN Security Council resolution threatening Syria with unspecified action if it did not cooperate with the inquiry. Syria has strongly denied any role.
Mehlis has asked to question six senior Syrian security officials, but disagreements about the venue and the legal framework have yet to be resolved.