The head of the U.N.-backed tribunal investigating the 2005 murder of former Lebanese prime minister Rafik Hariri has urged the four suspects in the case to appear before the tribunal, and stressed the proceedings will be conducted fairly.

Judge Antonio Cassese issued a statement Thursday saying it may be possible for the four men to appear by video instead of in-person, but that they should at least appoint lawyers.

The judge said Lebanese authorities have been unable to arrest the suspects, who are members of the Lebanon-based Hezbollah militant group. Hezbollah has denied involvement in the killing.

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