He said German investigator Detlev Mehlis on Tuesday quizzed Colonel Mustafa Hamdan after obtaining permission from Lebanese President Emile Lahoud.
Hamdan is the highest-profile pro-Syrian security official to retain his job after al-Hariri's assassination on 14 February.
UN staff went to Hamdan's house and office to pick him up.
Some politicians blame Damascus and the Lebanese security chiefs it allegedly backs for al-Hariri's killing and have called for Hamdan and the president to resign.
Lebanon's security chief, Jamil al-Sayyed, resigned in April as Syria bowed to international and Lebanese pressure to pull out its last military and intelligence units from its neighbour.
The international investigation, which got started late last week, was also in response to the demands of the anti-Syrian movement in Lebanon.