Story Highlights • Attackers drove SUVs, got past Iraqi checkpoint speaking English

• U.S. troops don't man Karbala checkpoint, Iraqi official says

• No Iraqis killed or wounded in attack, official says

• Attackers' SUVs found; two gunmen killed, U.S. says

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KARBALA, Iraq (CNN) -- Attackers who killed five U.S. troops at a government building in Karbala posed as U.S. military officials to get past Iraqi guards, a Karbala police spokesman said.

The attack happened Saturday as the U.S. military convened a meeting to discuss security for Ashura, the upcoming Shiite pilgrimage to Karbala.

According to police spokesman Abdul Rahman al-Mishawi, about 30 gunmen traveling in a convoy of at least seven SUVs with tinted windows -- similar to the vehicles used by top U.S. military officials -- drove up to the Karbala Provincial Joint Coordination Center wearing uniforms similar to those worn by the U.S. military.

About a dozen U.S. troops were inside the compound at the time, al-Mishawi said.

Around 5:45 p.m., the gunmen cleared an Iraqi police checkpoint outside the center by flashing fake identification badges and speaking some English, al-Mishawi said.

Al-Mishawi said it is standard procedure for U.S. troops not to jointly man the checkpoint. He said U.S. personnel insist on passing without going through a security screening.

The attackers went through three checkpoints to enter the center, he said.

The first U.S. casualty in the attack was a soldier sitting in a Humvee outside a meeting of U.S. and Iraqi security officials.

The assailants targeted only U.S. soldiers, al-Mishawi said, adding that not a single Iraqi soldier or police officer was killed.

Several of the SUVs used in the attack were found late Saturday in neighboring Babil province, along with two of the suspected gunmen, an official in Baghdad said. The official, who spoke on condition of anonymity, said the gunmen were wounded and detained by the Americans, but the U.S. military said the two were found dead.

When asked why Iraqi police did not intervene to stop the gunmen from fleeing, al-Mishawi said "they assumed it was American-on-American violence and wanted to stay out of it."

Al-Mishawi said Monday that "the Americans have shut down the provincial government compound and everyone is being interrogated from the police chief, officers, down to the average policemen."

The Karbala deaths made Saturday one of the deadliest days for the U.S. military since the war began almost four years ago, with 25 troops killed. Twelve of those troops died in a military helicopter crash. (Full story)

Ashura commemorates the martyrdom of Hussein, the grandson of the Prophet Mohammed. It attracts millions of pilgrims.

Karbala has two of the holiest shrines for Shiites -- the Imam Hussein and Imam Abu Fadhel al-Abbas.

In Baghdad on Monday, at least 75 people were killed and 160 wounded in a pair of nearly simultaneous car bombings in a used clothing market. (Full story)

CNN's Sam Dagher and Michael Holmes contributed to this report.