This combination of three images shows the stages of destruction of the Askariya Shrine in Samarra, Iraq. From top to bottom: a February 2004 photo of the shrine; the shrine in February 2006 after an explosion that destroyed its dome; and a photo from Wednesday after insurgents blew up its two minarets.

Baghdad ? Suspected al-Qaida bombers toppled the towering minarets of Samarra’s revered Shiite shrine on Wednesday, dealing a bold blow to Iraqi hopes for peace and reopening old wounds a year after the mosque’s Golden Dome was destroyed.

The attack stoked fears of a surge in violence between Muslim sects. Prime Minister Nouri al-Maliki’s government rushed to contain Shiite wrath against Sunnis: It clamped a curfew on Baghdad and asked for U.S. troop reinforcements in Samarra, 60 miles north of here, and for a heightened American military alert in the capital.

But sketchy reports of sectarian strife began to come in. Police told of at least four Sunni mosques in Baghdad and south of the capital attacked by arsonists and bombers, and of a smaller Shiite shrine bombed north of here.

The Samarra attack also threatened to deepen Iraq’s political crisis, as the 30-member bloc of radical Shiite cleric Muqtada al-Sadr immediately suspended its participation in parliament in protest.

The Golden Dome bombing in February 2006, at one of Iraqi Shiism’s holiest sites, was also blamed on Sunni militants linked to al-Qaida. That attack unleashed a bloodbath of reprisals – of Shiite death-squad murders of Sunnis, and Sunni bombing attacks on Shiites. At least 34,000 civilians died in last year’s violence, the United Nations reported.

Wednesday’s stunning attack came in near-simultaneous explosions about 9 a.m., completely bringing down the two slender golden minarets, 100 feet tall, that had flanked the dome’s ruins. No casualties were reported.

How the attackers evaded the Askariya shrine’s guard force, strengthened considerably after the 2006 bombing, was a mystery.

Al-Maliki said policemen at the shrine were detained for questioning – 15 of them, according to a senior U.S. military official. The prime minister also said an unspecified number of other suspects were arrested in Samarra and were being interrogated in connection with the shrine attack.

Neighbors alarmed

The blasts shook the Tigris River city of Samarra, sending a cloud of dust billowing into the air, said Imad Nagi, a storeowner 100 yards from the shrine. “After the dust settled, I couldn’t see the minarets any more. So, I closed the shop quickly and went home.”

Resident Abdul-Khali Mohammed predicted violence in the capital: “The Shiite militias now will seize this opportunity to kill Sunni families in Baghdad.”

An indefinite curfew was immediately imposed on Samarra, and, as Iraqi army and police reinforcements and U.S. troops poured in, the streets emptied by mid-afternoon.

In Baghdad, the prime minister ordered an indefinite curfew, beginning at 6 p.m. Wednesday, on vehicle traffic and large gatherings in the capital. Al-Maliki then traveled to Samarra with U.S. ground forces commander Lt. Gen. Raymond Odierno and visited the mosque ruins.

Al-Qaida blamed

An official close to the prime minister, citing intelligence reports, said Wednesday’s bombing was likely the work of al-Qaida, whose militants have recently moved into Samarra from surrounding areas.

A U.S. statement, from Ambassador Ryan Crocker and U.S. Iraq commander Gen. David Petraeus, unequivocally blamed al-Qaida, saying the terror group sought “to sow dissent and inflame sectarian strife.”

Such an attack by the Sunni extremist group al-Qaida in Iraq, increasingly at odds with more nationalist Iraqi insurgents, might have been intended to provoke Shiite retaliation that would help reunite various Sunni elements.

Petraeus told ABC News that that although he initially had a “terrible sinking feeling” after the attack, he believes there is reason to be optimistic in the fight against al-Qaida in Iraq.

“There is even some hope, perhaps, that al-Qaida may have overplayed its hand, as it did in Anbar Province, as it has in some neighborhoods in Baghdad, and in some other areas where, as you know, Sunni Arabs have rejected al-Qaida and have actually sought to join coalition forces and Iraqi governmental institutions to fight against it.”