Ayatollah Mohammed Baqer al-Hakim had returned to Iraq only three months ago. A long-time opponent of the ousted Iraqi leader Saddam Hussein, Ayatollah Hakim had been living in exile in Iran for more than 20 years.

After his triumphant return to the holy city of Najaf, the religious leader kept a relatively low political profile. But the group he led, the Supreme Council for the Islamic Revolution in Iraq, has taken an active role in Iraq's transition process.

That put him at odds with other Shi'ite factions struggling to be the voice of the country's majority population. It also made him a target for Baath Party loyalists of Saddam Hussein who oppose the U.S.-led occupation.Ayatollah Hakim's death comes four months after the murder of another Shiite cleric that was blamed on a intra-Shiite power struggle. It also comes a week after an assassination attempt against his own uncle, a prominent moderate Shiite leader.

Middle East analyst Kenneth Pollack says suspicion of guilt will fall on different factions, including Saddam supporters, al-Qaida terrorists and rival Shiite leaders who stand to gain from stirring up trouble.

"Baqer al-Hakim was a long-time foe of Saddam," said Mr. Pollack, who directs Middle East policy research at the Brookings Institution in Washington. "That would be reason enough for Saddam loyalists to go after him. He was a very important Shia figure. That's more than enough for al-Qaida to go after him. He's also someone who, even though he has kept the United States at arm's length, has demonstrated a willingness to cooperate with the U.S.-led reconstruction of Iraq. For all those reasons it's just too early to know for certain who was behind this."

The uncertainty, Mr. Pollack notes, could fuel a wave of unfocused violence, especially since the attack took place at one of the holiest Shiite shrines, which was partly damaged in the bombing.

" There's going to be a lot of anger in the Shia community for the moment because no one does know who was responsible," he said. "That anger seems very unfocused."

That spreading anger, Daniel Serwer says, will intensify the security challenge for U.S.-led occupation forces.

"It puts more pressure on them to provide security in areas where, clearly, security can only be provided by Iraqis. And Najaf, in particular, is a holy city and clearly it would be preferable to have Iraqis dealing with the security situation," said Mr. Serwer, who directs peace operations for the U.S. Institute of Peace.

Angry Iraqis also have criticized the U.S.-led occupation forces for not protecting the Shiite leader.

But Iraqi-born political activist Laith Kubba says it is more than just a question of security.

"Yes, it will put tremendous pressure on America on delivering security. But the real dilemma is there is no real quick fix to security," said Mr. Kubba, who heads the Washington-based Iraq National Group that is trying to help map his country's political transition. "This has to be done within the context of better planning and better political management to the country. Pouring more men and more dollars is not going to fix or solve the problem. There is no substitute to having a sound plan on transition and this has been lacking."

The U.S. administration has recognized the urgency of strengthening its security operations, especially after the recent attacks on the United Nation's and Jordanian missions in the Iraqi capital that have spurred many foreign humanitarian missions to reduce their presence.