Unknown attackers destroyed the twin minarets of one of Iraq’s most holy Shiite shrines Wednesday morning, touching off protests, condemnation and fears of sectarian reprisals.

An attack last year at the same Samarra shrine — known as both Askariya and the Golden Mosque — destroyed its golden dome and touched off months of Shiite-Sunni violence that some called a civil war.

No injuries were reported in the 9 a.m. attack that destroyed the minarets, but security throughout the country was immediately tightened. Curfews went into effect in Baghdad and many other cities, Iraqi and American troops were put on high alert, and the entire Iraqi police unit charged with guarding the shrine was detained for questioning.

Shiite religious leaders immediately condemned the bombing but called for calm. Grand Ayatollah Ali al-Sistani, the most powerful Shiite cleric in Iraq, called on Iraqis to show “restraint.”

The White House said Wednesday that U.S. officials in Iraq will use lessons from last year’s bombing of a revered Shiite shrine in Iraq to try to contain a new wave of violence now that the mosque has been attacked again, The Associated Press reported.

Presidential spokesman Tony Snow said it’s not clear yet who perpetrated the attack early Wednesday on the shrine.

Radical Shiite cleric Muqtada al-Sadr — whose militia has been accused of running sectarian death squads in the wake of the first shrine attack — also called for peaceful protests and a three-day mourning period.

It was unclear Wednesday exactly what destroyed the minarets. While one Iraqi television station reported the damage was caused by two mortar rounds, several U.S. and Iraqi military officials said it appeared explosive charges had been planted in the compound.

The U.S. military issued only a four-line press release on the attack, saying that “the Iraqi Police on site described hearing two near simultaneous explosions coming from inside of the Mosque compound, but they did not see any attackers in the vicinity. The cause of the explosions is being investigated by the Iraqi police at the scene.”

A joint statement issued by U.S. Ambassador to Iraq Ryan Crocker and Gen. David Petraeus, the top U.S. commander in Iraq, condemned the “vicious attack.”

That statement called the attack a “deliberate attempt by al-Qaeda to sow dissent and inflame sectarian strife among the people of Iraq. It is an act of desperation by an increasingly beleaguered enemy seeking to obstruct the peaceful political and economic development of a democratic Iraq.”

Since the 2006 attack, the shrine had been guarded mainly by a predominately Sunni police unit. Samarra itself — thanks in large part to sectarian cleansing in the aftermath of the first attack — is almost completely Sunni.

But several reports Wednesday said that a new, Shiite unit had been tapped to replace the current police. Officials said the Sunni unit had reportedly been infiltrated by al-Qaida in Iraq operatives.

According to The Associated Press, a prominent Sunni cleric went on Al-Jazeera television and said the new unit had arrived chanting sectarian slogans, possibly provoking local Sunnis.

Government officials in Iran — a predominantly Shiite country — on Wednesday blamed the American military for not providing enough security at the shrine.

“You, by enabling these activities, will be cornered,” hard-line President Mahmoud Ahmadinejad said in a public speech, referring to U.S. troops in Iraq.

A spokesman for Iran’s foreign ministry said “occupier forces” — Iranian government shorthand for American troops — were responsible for not protecting the shrine.

U.S. officials have long accused Iran of providing arms and training to Shiite insurgents and militias in Iraq.

The Askariya shrine is commonly referred to as one of the four most important Shiite mosques in Iraq. Before the Feb. 22, 2006, attack, around a million Shiite pilgrims visited the shrine every year to see the graves of Islamic religious figures.

According to Islamic beliefs, the shrine is also where the 12th Imam — Mohammed al-Mahdi — was put into a state of “divine hiddenness” by God to protect his life. Shiites believe that the Mahdi will return at the end of days to bring justice to the world.

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