The murder of a leading Iraqi cleric by a horrific car bomb on Friday was more than a tragic blow to Iraqis.

It was a deadly challenge to the whole Iraq policy of the United States.

There was a ghoulish strategy apparent behind the bomb that killed Ayatollah Mohammed Bakr al-Hakim and dozens of worshippers outside Shiite Islam’s holiest shrine in Najaf. For that reason, this outrage generated even more fear than last week’s attack on U.N. headquarters in Baghdad.

The bombers want to show that the United States can’t restore stability to the country. They want to prove that mighty America can’t prevent the murder of any Iraqi. They clearly hope to turn Iraq’s Shiite majority — which has tolerated U.S. occupation — against the Americans.

And they want to sow fear that, if the occupation continues, Iraq will sink into a religious civil war.

The biography of the murdered man shows that the killers chose a target guaranteed to shock society.

One of Iraq’s most prominent clerics, from a leading religious family, Hakim had suffered torture and imprisonment under the Saddam Hussein regime, which murdered many of his family members. He returned from years in Iranian exile in May and was a moderating influence on Shiite politics. He had ties both to the Shiite clerical establishment, and, indirectly, to the U.S. authorities through his brother, Abdul Aziz, a member of the U.S.-appointed Iraqi Governing Council.

I interviewed Hakim twice in Tehran, once after the Persian Gulf War in May 1991 and again in February. His position had shifted from 1991, when he endorsed an Islamic state.

This year he told me he accepted a government “based on elections and respecting the particularities of all Iraqis, including Shia, Sunnis and Kurds.” We’ll never know for sure, but I think he meant it.

Wrapped in a long black robe and black turban in his spartan Tehran office, the cleric said he would accept a secular Iraq so long as it wasn’t “against Islam.” Once back in Iraq, he seemed more independent of Iranian influence than U.S. officials had expected, and bent on reassuring Iraqis that a tolerant state could emerge after Hussein’s tyranny.

His last sermon, just before his murder, exhorted fellow Iraqis to embrace “unity.”

The murder of such a prominent figure makes Iraqis feel that no one is safe from assassination — especially since, last week, another bomb wounded Hakim’s uncle, Ayatollah Mohammed Saeed Hakim, one of the four leading religious scholars in the Shiite religious establishment in Iraq.

Shiites are angry that U.S. forces did not prevent these bombings. Never mind that the Marines stayed away from the holy shrines for fear of offending. Hakim’s aides are already bitterly complaining that they had asked, and were not allowed, to set up a special force to protect the shrines.

So who plotted this crime? Every Iraqi I asked here and in calls to Baghdad gave the same answer: remnants of Hussein’s secret police forces.

No one seems willing to believe that any Shiite would kill so famous an ayatollah beside the holy shrine. This reasoning is even being applied to the thuggish young radical cleric Muqtada al-Sadr, whose followers murdered the son of another famous cleric in April — and also threatened Hakim supporters. For the same reason, Iraqis are so far discounting any plot by Iranian security forces, who are also Shiites.

But I was told repeatedly that Hussein’s supporters may be trying to stir up a religious civil war between Shiites and Sunnis. I spoke by phone with Muhyi al-Khateeb, one of the smartest Iraqi analysts I know, who is secretary general of the Iraqi Governing Council. He talked as he stood with hundreds of mourners lined up to convey condolences to Hakim’s brother in the garden of his Baghdad home.

Khateeb told me bluntly, “I think now they (the Hussein remnants) will kill a Sunni cleric and blame it on Shia shoulders and drag us to a bloody civil war.” In other words, the assassins may try to spark a religious war of revenge.

So what is to be done to thwart the strategy of the killers?

The Bush team has little time to play catch-up. U.S. officials must do whatever is necessary to close borders, track the terrorists, and protect leading Iraqis — and shrines.

Khateeb thinks U.S. officials should reconstitute elements of the Iraqi army that they recently abolished en masse and use them as local police forces. He said, “Costa Rica demobilized its army (in 1948) but brought 40,000 to 50,000 back and made them into police.” In a dig at U.S. policy, he added, “The Costa Ricans prepared ahead. They didn’t demolish a huge army and make them into enemies without bringing up a new police force.”

Khateeb says, and I agree, that the United States must give Iraqis more authority to run the country and to police themselves ASAP. Is it too late? I asked. “No, it is not too late,” he replied. “It is either now or never, either now or we lose everything.”