Why has he not been handed over for questioning by the commission of inquiry headed by Detlev Mehlis?" asked Haitham al-Mallah, former chairman of the Organization for Human Rights in Syria.
Ziad Ramadan, who is of Syrian nationality, had been able to receive a prison visit from his family two weeks after his arrest, Mallah said.
Ramadan is named in the interim report released by the UN inquiry last Thursday as best friend of Ahmed Abu Adass, the Lebanese Islamist who made the videotape claim on the day of Hariri's assassination on the Beirut seafront.
The commission made clear it did not believe Abu Adass was actually responsible for the February bombing but expressed interest in his connections with Syria and the apparent disappearance there of both him and his best friend.
"The sudden return to Syria of Mr Abu Adass's Syrian best friend Ziad Ramadan shortly after being interviewed by the Lebanese authorities ... suggest shadowy Syrian connections to Mr Abu Adass's disappearance," its report said.
The accusation that Syria was holding Ramadan without making him available to the UN commission of inquiry came as the Security Council prepared to meet to consider Mehlis's report implicating top Syrian officials and accusing them of insufficient cooperation with its work.