BAGHDAD -- Two explosions that appeared to have been set by Sunni extremists with al-Qaida links toppled yesterday the twin golden minarets that were most of what remained of one of Iraq's most revered Shiite shrines after a devastating bombing by al-Qaida last year.
The bombing 16 months ago proved a watershed, engulfing the country in a wave of sectarian killing that pushed Sunnis and Shiites toward civil war.
With American and Iraqi forces unable to restrain soaring levels of killing that saw as many as 3,000 Iraqi civilians dying every month by the end of last year, President Bush ordered nearly 30,000 additional American troops deployed here in a so-called surge aimed at pulling the country back from the abyss.
But after yesterday's renewed attack on the shrine at Samarra, 75 miles north of Baghdad, appeals for calm by Shiite political and religious leaders, as well as by moderate Sunni politicians and the top two American officials in Iraq, appeared to have headed off the risk of a new sectarian convulsion, at least for now.
By nightfall, with emergency curfews in Baghdad and other cities, and Iraqi forces moving in to protect Sunni and Shiite mosques across the country, there were only scattered reports of reprisal attacks.
The new attack on the shrine came at what American commanders acknowledge to be a now-or-never point in the war here, with only months for the fresh troops deployed to begin driving down insurgent attacks.
Without significant progress by September, when the top American military commander here is scheduled to report to the president and Congress, the generals appear to have little prospect of holding off congressional and popular pressures at home for an American troop withdrawal.
The Samarra bombing appeared to be aimed at derailing the American hopes for a turnaround in the war by triggering renewed Shiite reprisals, refocusing the conflict on Sunni-Shia tensions and away from recent infighting between Sunni tribal groups and al-Qaida-linked Sunni insurgents.
Encouraged by the split, the Americans are adopting a strategy of arming Sunni Arab groups that promise to fight the al-Qaida militants, a tactic that has worked in Anbar province, but that also risks arming both sides in a civil war.
The explosions that destroyed the 120-foot-high minarets of the Imam al-Askari shrine occurred about 9 a.m.
A statement by the American military command, quoting Iraqi police, said that they "did not see any attackers in the vicinity," suggesting that the bombings were an inside job.
Suspicion fell immediately on the guard force that was protecting the shrine, a unit of a few dozen local men composed almost of entirely of Sunnis, the population group that predominates in Samarra and has controlled the shrine, through Iraq's Sunni clerical establishment, since it was built in 1905.
American and Iraqi commanders in the area had suspected the guard force of harboring al-Qaida sympathizers, but they were unable to dislodge them.
Iraqis shopping in a local market said they heard the explosions, ran to look toward the shrine, and saw as a cloud of dust and debris cleared that the minarets, used traditionally as the place from which the Muslim faithful are called to prayer, had vanished.
"Where the minarets had been, there was nothing," said one man who asked not to be named. "They had just disappeared."
The bombings left the shrine, its soaring golden dome destroyed in the initial attack in February last year, with little left of its former grandeur but a mound of rubble overlooked by a huge blue-tile archway and a golden clock tower.
The mosque is atop what is said to have been the burial site, in the ninth century, of two of the 12 imams, the apostles of the Shiite faith. Shiites believe both imams to have been poisoned on the orders of a Sunni caliph in Baghdad.
Prime Minister Nouri al-Maliki, a religious Shiite who leads a fractious and increasingly dysfunctional coalition government of Shiites, Sunnis and Kurds, said in a hastily prepared television address from his office in Baghdad that he had ordered the arrest of "all officials who were responsible for guarding the holy shrine."
Reports from Samarra said that at least 15 members of the guard force, most of them nominally employees of the Interior Ministry in Baghdad, were taken into custody.