Mahmud Abd al-Al, a member of the pro-Syrian al-Ahbash Muslim group, was detained in Beirut early on Saturday on orders from Lebanese Magistrate Elias Eid.
His arrest was the first since chief UN investigator Detlev Mehlis released his findings into the investigation of al-Hariri's 14 February murder in a Beirut car bombing that killed at least 20 others.
The officials, who spoke on condition of anonymity, said Eid based his decision to detain Abd al-Al on a recommendation from top UN investigators.
Mehlis's report alleged that Abd al-Al called pro-Syrian Lebanese President Emile Lahoud on his mobile telephone minutes before the blast that killed al-Hariri. Lahoud denied receiving such a call.
Lahoud denied receiving a call
from Mahmud Abd al-Al
The UN inquiry said that shortly after, Abd al-Al also contacted one of four Lebanese pro-Syrian generals who have since been arrested in the inquiry.
Police also seized unspecified documents during the raid on Abd al-Al's home, the officials said without elaborating.
Abd al-Al's brother is a prominent figure in the al-Ahbash group, Ahmad Abd al-Al, whom Mehlis identified as a "key figure" in the ongoing investigation.
Ahmad Abd al-Al had extensive contacts with top Lebanese security officials before and after the blast, and tried to hide information from investigators, according to the UN report.
He was recently arrested in Beirut in connection with a weapons depot discovered in southern Lebanon in July.