No formal charges have been filed, but the probe signals that Iran's Islamic leadership could use the courts against the most senior dissenters — whose claims include widespread vote fraud in June's disputed presidential elections and abuses by security forces against protesters in the violent aftermath.
The powerful Revolutionary Guard and other pro-regime groups have urged for charges against the top opposition figures, including Karroubi, fellow reformist candidate Mir Hossein Mousavi and former President Mohammad Khatami.
More than 100 activists and political figures have already been tried on charges that include seeking to overthrow the Islamic system.
The official Islamic Republic News Agency quoted Tehran prosecutor Abbas Jafari Dolatabadi as saying an investigative file has been opened on Karroubi and the case is under review by a special cleric court.
Dolatabadi said "some individuals have already been summoned" as part of the inquest in Karroubi — a Shiite cleric and former parliament speaker — and added that 10 more people would go on trial on charges related to the postelection turmoil. But the report gave no names or further details.
Iran's judiciary is firmly under control of the ruling Judicial authorities and could not be immediately reached for comment.
But it's likely the investigation would include Karroubi's explosive accusations of abuses by security forces. Karroubi has led claims of rape and torture against protesters detained during the demonstrations and clashes following President Mahmoud Ahmadinejad's re-election.
Karroubi has demanded high-level investigations into the abuse claims. Karroubi's newspaper, Etemad-e-Melli, was banned by authorities in August.
Parliament Speaker Ali Larijani has denied the allegations of rape and abuses by security forces, saying a parliament probe into the allegations had found no truth in the reports.
The judicial probe could be an attempt at further pressure to muzzle opposition. Any effort to bring charges would open a new front by authorities to destroy the top ranks of the opposition movement with possible serious charges of "propagating against Islamic system" — which can bring long prison terms.
The Special Cleric Court operates in heavy secrecy and its proceedings are closed to the public.
Iranian authorities have faced international condemnation for the crackdowns following the June election, but attention has greatly shifted to the regime's negotiations with Western powers over its nuclear program. Another round of talks is planned for later this month