The leader of the Supreme Council for the Islamic Revolution in Iraq (Sciri), Ayatollah Muhammad Baqr al-Hakim, was among about 100 people killed in a massive car bombing in the Shia holy city of Najaf in August 2003.
Ayatollah Hakim opposed Saddam Hussein from exile in Tehran before returning in May 2003 to widespread acclaim from thousands of Iraqi Shias.
Before his death, the ayatollah offered qualified support to the US-appointed Governing Council – giving the body much-needed legitimacy among Iraqi Shias.
He had many supporters in Iraq, but his close connections to the ruling clerics in Iran made the US - and some Iraqis - wary of him. But analysts say that as his stance shifted towards the US, his backing in Iran became weaker among Iranian conservatives but stronger among reformists.
Ayatollah Hakim's brother, Abdel Aziz, has taken over as leader of Sciri. He has carried out negotiations with the US and represented Sciri at international meetings, and holds Sciri's only seat on the US-appointed Iraqi Governing Council. He also has close ties with Iran.
The organisation has an armed wing funded by Iran, the 10,000-strong Badr Brigade.