The U.N.-backed tribunal investigating the 2005 assassination of former Lebanese Prime Minister Rafik Hariri has widened its probe to include attacks on three other politicians that it believes are linked.

The tribunal said Friday that it will look into the 2005 killing of former Communist Party chief George Hawi. The group will also investigate a 2004 assassination attempt against former telecommunications minister Marwan Hamadeh and a 2005 attempt against former Deputy Prime Minister Elias Murr.

The special court says Judge Daniel Fransen issued a ruling that will allow the tribunal to investigate the three attacks after prosecutors presented evidence showing similarities between the attacks and Mr. Hariri's killing.

Friday's announcement comes two days after the tribunal released its indictment against four Hezbollah members who prosecutors say carried out the attack against Mr. Hariri.

Former Lebanese Prime Minister Saad Hariri welcomed the court's decision to unseal the indictment related to his father's murder. But Hezbollah leader Hassan Nasrallah dismissed the indictment, saying it contained “no direct evidence” against the suspects.

Mr. Hariri and 22 others were killed in a massive truck bombing in the Lebanese capital, Beirut.

Hezbollah has denied involvement in the attack.