Lebanon's chief prosecutor has issued a summons for Libyan leader Muammar Gaddafi to appear for questioning over the disappearance of a Lebanese cleric.
Sadr was the head of Lebanon's Shia community
Moussa Sadr, the spiritual leader of Lebanon's Shia community, disappeared while visiting Libya in 1978.
Mr Sadr's son filed a suit in Beirut last month against Mr Gaddafi and 17 other Libyans.
Lebanese officials say they have new information about people allegedly involved in Mr Sadr's disappearance.
Two of Mr Sadr's assistants, Sheik Mohammed Yacoub and Abbas Badreddine, disappeared with him.
Their wives joined Sadreddine Sadr in filing the complaint.
A lawyer for the families said the suit contained "claims that clarify that some people have taken part in this crime".
The Prosecutor-General, Adnan Addoum, said former Libyan Prime Minister Abdel Salam Jalloud and a former Libyan ambassador to Lebanon should also be summoned.
The families praised Mr Addoum's decision, calling it a "victory for justice in a crime that has been going on for more than a quarter of a century", the AP news agency reported.
In a statement, they said they would continue to do whatever they could to "reveal the full truth and punish Muammar Gaddafi and his accomplices in the crime".
Lebanon claims Mr Sadr disappeared after an argument with Mr Gaddafi.
Libya says the cleric boarded a plane for Rome, but the Italian authorities denied that he ever arrived.
The on-going row has soured relations between the two countries.
Libya closed its embassy in Beirut last year, claiming that it was insulted by Lebanese pressure to reveal Mr Sadr's fate.
Mr Gaddafi did not attend an Arab summit in Beirut in 2002 after he was threatened by Lebanese Shia groups who claimed Libya was responsible for Mr Sadr's disappearance.
Born in Iran, Mr Sadr was an influential figure in Lebanese politics.
A charismatic speaker and religious scholar, Mr Sadr is credited with helping transform the country's Shia Muslims into a major political force.
He founded the populist Amal movement, which became the largest Shia organisation in Lebanon and played an important role in the country's 15-year-long civil war.
The Syrian-backed group with political and military wings was also active against Israel's 22-year-long occupation of the south of the country.
After the civil war, Amal passed into the hands of more secular leaders. Its current leader, Nabih Berri, is the speaker of the Lebanese parliament.