Speaking on the second day of the trial before a UN-backed court in The Hague, defence lawyers said the prosecution's allegations over who killed billionaire Hariri and 22 others in the February 15, 2005 car bombing were a "mere theory."
"It's quite surprising to see this crime bereft of motive for the time being," Vincent Courcelle-Labrousse told journalists.
"The prosecutor has not put forward any reason" for the bombing, he said after Special Tribunal for Lebanon (STL) prosecutors wrapped up their opening statements.
Cameron laid out details of how the killers allegedly used mobile phone networks to conduct surveillance of Hariri until minutes before the explosion.
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"There is nothing tangible, there is no phone tapping, no witnesses, no text messages, there is nothing substantial here."
"We still don't know who killed Rafiq Hariri."
Hariri, Lebanon's Sunni prime minister until his resignation in October 2004, was on his way home for lunch when a suicide bomber detonated a van full of explosives equivalent to 2.5 tonnes of TNT as his armoured convoy passed.
A fifth suspect, Hassan Habib Merhi, 48, was indicted last year and his case may yet be joined to the current trial.
The four suspects have been charged with nine counts, ranging from conspiracy to commit a terrorist act to homicide and attempted homicide.
Prosecutors allege that Badreddine and Ayyash "kept Hariri under surveillance" before the Valentine's Day suicide bombing, while Oneissi and Sabra allegedly issued a false claim of responsibility to mislead investigators.