The killing of Ayatollah Mohammad Baqr al-Hakim has added new urgency to the question that surfaced when Saddam Hussein's regime collapsed in April: Who speaks for the Iraqi Shia?
Comprising some 60% of the Iraqi population and convinced they have long been denied a fair share of power, the Shia are a factor no-one can ignore.
Shias blame Saddam Hussein's supporters for the attacks
They will need to be represented fairly in the new Iraq. But how?
The Shia are divided between the secular and the religious, and their clergy are divided between activist and "quietist" traditions.
These two traditions compete for influence within the Hawza, or seminary, in the holy city of Najaf in the Shia heartland of southern Iraq.
The Hawza is the main source of authority for the Iraqi Shia.
The Hawza's most senior figure, Ayatollah Ali Sistani, follows the quietist trend. In other words, he believes the clergy should provide spiritual leadership but, as far as possible, steer clear of politics.
But even he has felt the need to say something about the central issue of the day - whether to work with the country's American-led administration or boycott it.
The motive may have been to sow fear and division among the Shia, and provoke tension between the country's Shia majority and Sunni minority
Ayatollah Sistani's message to the Iraqi Shia is: "We should co-operate with the Americans, and wait to see whether they will do what they have promised - and hand over power to the Iraqi people."
That is the view of most of the senior clerics, and it was also the view of Ayatollah Mohammad Baqr al-Hakim, the religious and political leader killed in Friday's car bomb attack.
The radical challenge
But it is a view that has come under strong attack from a more militant faction led by a radical young Shia, Muqtada al-Sadr.
The son of a popular ayatollah - killed, most Shia believe, by agents of the Saddam regime - Muqtada al-Sadr urges Shias to oppose the occupation.
He hints at the need for resistance, while stopping short of open incitement to violence.
The Sadr group has been blamed for earlier acts of violence. In April a prominent cleric, Abdel-Majid al-Khoei, who had returned from exile in Britain, was killed by an angry crowd in Najaf.
Ayatollah Hakim's death has intensified the crisis of leadership confronting the Iraqi Shia
In late August, there was an unsuccessful attempt to assassinate Ayatollah Hakim's uncle, also a prominent cleric.
Muqtada al-Sadr has vigorously denied involvement in these attacks or in the killing of Ayatollah Hakim.
But there can be no doubt that his angry criticism of senior clerics, whom he has denounced for passivity in the face of foreign occupation, has contributed to the mood of tension and anxiety within the Shia community.
Ayatollah Hakim's death has intensified the crisis of leadership confronting the Iraqi Shia.
It has weakened his movement, the Supreme Council for the Islamic Revolution in Iraq (Sciri).
He founded Sciri in Iran, after fleeing Saddam's persecution, and ran it from Tehran for more than 20 years before returning home earlier this year.
Hakim's younger brother, Abd-al-Aziz, is one of the 25 Iraqis chosen by the Americans in July to form the Iraqi Governing Council - an interim body designed to pave the way for an elected government.
Questions are being asked about security flaws that led to the murder of the cleric
But Abd-al-Aziz lacks his brother's authority.
Most Iraqis believe Ayatollah Hakim was killed by Sunni Muslims loyal to the old Saddam regime or by Sunni militants who have entered the country from outside.
Religious leaders are warning against the danger of sectarianism.
Whoever carried out last week's car-bomb attack, the motive may have been to sow fear and division among the Shia, and provoke tension between the country's Shia majority and Sunni minority.
The need for leadership could hardly be greater.