Rashid Mizhir, a military magistrate, ordered the arrest of Abd al-Qadir Abd al-Qadir, a scrap dealer, after questioning him and two other Syrians who have since been released, judicial sources said on Tuesday, on condition of anonymity.
The 12 December car bomb killing of Tueni was the third political murder in Lebanon since the 14 February assassination of former premier Rafiq al-Hariri plunged the country into months of turmoil.
Abd al-Qadir was present near the site of Tueni's killing in a Christian suburb of Beirut and made mobile phone calls before and after the explosion, which also killed Tueni's driver and bodyguard, the sources said without elaborating.
An ongoing UN investigation has implicated senior Syrian officials and their Lebanese allies in al-Hariri's murder, echoing feelings among many Lebanese who also blame Damascus for the other political killings.
Syria denies any role, but al-Hariri's assassination sparked mass protests in Beirut that forced it to bow to world pressure and withdraw its troops from Lebanon in April after 29 years.
Detlev Mehlis, the outgoing chief of the inquiry, said earlier this month al-Hariri's murder was probably linked to other politically-motivated crimes.
The 48-year old Tueni was a fierce critic of Syria's grip on Lebanon, publishing his biting editorials on the front-page of his al-Nahar newspaper, Lebanon's leading daily.