The Special Tribunal for Lebanon said Monday it transferred the case to the trial chamber.
"The transfer of the case file is a significant development in the proceedings ... and it marks the start of the transition from the pre-trial to the trial phase," it said Monday.
Suspected Hezbollah members Salim Jamil Ayyash, Mustafa Amine Badreddine, Hussein Hassan Ainessi and Assad Hassan Sabra are on trial in absentia. The special tribunal issued an appeal to help find Hassan Habib Merhi, a fifth suspect not tied formally named in the case.
Hezbollah denies ties to the assassination and says the matter is part of an Israeli plot. Badreddine was identified as the brother-in-law of Hezbollah commander Imad Mughniyeh, who was killed in 2008.
Hariri was among 22 people killed in a bombing attack Feb. 14, 2005, in Beirut.
The tribunal said the trial against the four original suspects is scheduled tentatively for Jan. 13.