WASHINGTON—The commander of the Iraqi battle space, General David Petraeus, is trying to determine how high within the Iraqi Security Forces a conspiracy rose to place a bomb inside one of the holiest sites in Shiite Islam, the Askariya Mosque in Samarra.

So far, cash payments and other pieces of intelligence have suggested varying degrees of complicity for some senior officers within Iraq’s national police, according to an American military officer who requested anonymity. The officer said the Tikriti police colonel who was in charge of one of the units guarding the mosque is in custody and being questioned.

Major General Benjamin Mixon told CNN that he believed yesterday’s bombing was an inside job. General Petraeus and Prime Minister al-Maliki are now scrambling to contain an expected spike in sectarian violence in the aftermath of the second bombing in 16 months of the Askariya Mosque.

The Iraqi government and the American military yesterday imposed a total ban on vehicle traffic in Baghdad and urged Iraqis to refrain from revenge killings. Mr. Maliki announced on Iraqi television that a curfew was in effect until further notice. Meanwhile, General Petraeus on CBS News called the attack a “serious blow.” In the same interview he said the Army was taking advantage of recent intelligence culled from an Al Qaeda safe house. “We just recently hit one of their safe houses, we are still exploiting the materials that were taken from that,” he said, adding that he believed the perpetrators were linked to the organization that felled the twin towers on September 11.

Al Qaeda operatives took credit for the first bombing of the Askariya Mosque, which destroyed its golden dome on February 21, 2006. President Bush has blamed that attack on a wave of sectarian killings in Iraq the following year. Two military sources yesterday said that four Sunni mosques had been destroyed since the bombing; two mosques in Baghdad, one mosque in Basra and another in Diyala.

The blast yesterday in Samarra, which took down the Askariya Mosque’s two minarets could cause the beginning of the end of the American campaign in Iraq. The mosque is a shrine to Shiism, the faith of at least 60% of Iraqis, and its bombing is a powerful symbol that neither the Iraqi government nor the American forces in Iraq can protect civilians.

Should the mosque incident incite a new round of revenge executions, America’s war weariness could in turn deepen. Already some national polls from May show more than 60% of Americans favor a withdrawal deadline for Iraq sometime in 2008. The speaker of the House, Nancy Pelosi, and the Senate majority leader, Harry Reid, both Democrats, yesterday sent a letter to President Bush laying the groundwork for a withdrawal deadline to be included in the next bill to fund the troops in Iraq.

The leader of the Shiite militia responsible for most of the killing targeting Sunnis, Moqtada al Sadr, yesterday issued a statement from Najaf urging his followers to observe three days of mourning after the attacks. But a spokesman for Mr. Sadr yesterday told Arab news outlets that the blame for the attack lies with the “occupiers,” a direct appeal to Iraq’s Shiites to stop cooperating with the Baghdad security plan and American soldiers.

That line echoed words from Iran’s president, Mahmoud Ahmadinejad, who blamed America for the Samarra blast. “You, by enabling these activities, will be cornered,” he threatened. American generals have blamed Iran for supplying weapons to both Sunni and Shiite terrorists in Iraq.

In contrast to Mr. Sadr and Mr. Ahmadinejad, the leading clerical authority in all of Shiism, Ayatollah Ali Sistani, yesterday urged “believers to exercise self-restraint and avoid any vengeful act that would target innocent people or the holy places of others,” according to the Associated Press.

Even some of the war’s staunchest supporters yesterday conceded that the bombing in Askariya could quickly accelerate sectarian killings. A former chairman of the Pentagon’s Defense Policy Board, Richard Perle, said, “If it has the same reaction as the initial bombing it will be disastrous.” But Mr. Perle said we did not yet know if Iraqis would react the same way and pointed out that the White House has carefully tried to dampen expectations for the summer.

A professor of strategic studies at the Johns Hopkins University School for Advanced International Studies, Mary Habeck, yesterday said she thinks the implications of the mosque bombing won’t be the same as last year. “If this was last year and this was the first time this happened, I would think this was the end of the world,” Ms. Habeck said. She added that Iraqis have the bitter experience of turning to al Qaeda and Mr. Sadr’s Mahdi Army in the last year. “When people ran to them for support and protection, they found themselves being forced to live in a way that does not match with their understanding of Islam, and they found themselves being killed and intimidated by al Qaeda,” she said.