BAGHDAD - Gunmen wearing military uniforms over explosives belts charged into a government building in Saddam Hussein's hometown Tuesday in an attack that left 56 people dead, including 15 hostages who were shot execution-style.

The five-hour standoff in Tikrit ended only when the attackers blew themselves up in one of the bloodiest days in Iraq this year.

American troops who were nearby as part of an advising mission with Iraqi forces responded to the attack, and some US soldiers received minor wounds, said military spokesman Col. Barry Johnson. The US troops dropped back after Iraqi forces took control, Johnson said.

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The assault was reminiscent of the bloodshed that was common during the worst days of the conflict as Iraq was pushed to the brink of civil war. Attacks have ebbed in recent years, but the looming deadline for the US troops to withdraw from the country along with political unrest elsewhere in the Mideast have raised fears the country could return to violence.

The standoff in Tikrit, the capital of Salahuddin province 80 miles (130 kilometers) north of Baghdad, began around 1 pm when the attackers blew up a car outside the council headquarters to create a diversion before launching their raid.

Wearing military uniforms - including one with a high rank - the gunmen identified themselves as Iraqi soldiers at a security checkpoint outside the government compound but opened fire on guards when they were told they needed to be searched.

They later set fire to the bodies of the three slain provincial councilmen in a defiant show of how insurgents maintain the ability to carry out brutal attacks despite years of US-Iraqi military efforts against them.

"Why did they shoot him and set fire to his poor body?" said Salahuddin government spokesman Mohammed al-Asi, trying not to weep when confirming the killing of lawmaker Mehdi al-Aaran, an elderly man who headed the council's religious affairs committee.

Speaking in a muted voice, Salahuddin Gov. Ahmed Abdullah called the attack "a tragic incident carried out by ruthless terrorists."

Iraqi officials were quick to blame al-Qaida in Iraq for the slaughter, noting that execution-style killings and suicide bombers are hallmarks of the extremist group. A senior intelligence official in Baghdad likened the attack to al-Qaida's hostage raid last fall on a Catholic church in Baghdad that left 68 dead and stunned the nation.

Tuesday's attack left 56 people dead and 98 wounded, including government workers, security forces and bystanders, said Salahuddin health director Dr. Raied Ibrahim. Many died in the volleys of gunfire and explosions.

Among the dead were councilman Abdullah Jebara, a vocal al-Qaida foe; the council's health committee chairman, Wathiq al-Sammaraie; and Iraqi journalist Sabah al-Bazi, a correspondent for Al-Arabiya satellite TV channel and a freelancer for CNN and Reuters.

Members of Iraq's parliament immediately called for an investigation into how the band of eight or nine insurgents could pull off the attack and paralyze a mostly Sunni Muslim city that was once a hotbed for al-Qaida in Iraq and Saddam sympathizers.

Officials are particularly sensitive about the ability of Iraqi security forces to protect the country as US troops plan to leave at the end of the year.

"We denounce this sorrowful act, where insurgents with military uniforms could break into the council building," said parliamentarian Suhad al-Obedi, who represents Salahuddin province. "This is a security breach."

It's not hard to buy uniforms on Iraqi streets, and the ease and deadliness of the attack demonstrated sophisticated planning by the gunmen.

"The gunmen were armed with grenades and began their raid by firing at random at a reception room," said Ali Abdul-Rihman, a spokesman for the governor. "Then they opened fire inside."

The provincial council meets at the headquarters every Tuesday, but Abdul-Rihman said local lawmakers ended their discussion early because there was little on their agenda. As a result, he said, most of the lawmakers had already left the headquarters when the assault began.

Al-Asi, the provincial spokesman, said 15 people were taken hostage on the headquarters' second floor, where the gunmen hurled grenades and fired at security forces below. The hostages, including three lawmakers, were each shot in the head, al-Asi said.

Parliament lawmaker Qutayba al-Jabouri said security forces did not try to negotiate with the gunmen since they were under assault. Gov. Abdullah described a fierce shootout between the gunmen and Iraqi security forces who surrounded the building.

Baghdad University political analyst Hassan Kamil called Tuesday's attacks "another indication that the insurgents are no way thinking of giving up the struggle in Iraq."

"It is a show of force aiming at convincing people that despite the setbacks, the insurgency is still active," Kamil said. "Security is still fragile."