It is an unprecedented trial in The Hague. Four men accused of murdering the former Lebanese Prime Minister Rafik Hariri have not been arrested and are being tried in absentia.
The judge said all necessary steps had been taken to try and bring the accused to court.
Members of the Hariri family were in court for the start of the tribunal including Saad Hariri, son of the deceased.
“Protecting the accused of the crime of killing Rafik Al Hariri and his companions and then not handing them over to justice is another crime on top of the main crime,” he said.
Warrants for the four were issued in 2011 after the UN set up the Special Tribunal for Lebanon in 2007 to investigate the bombing in which Hariri died.
The accused are allegedly members of Hezbollah. The Shia militant movement denies any involvement.