BAGHDAD, Iraq — In perhaps the boldest and most sophisticated attack in four years of warfare, gunmen speaking English, wearing U.S. military uniforms and carrying American weapons abducted four U.S. soldiers last week at the provincial headquarters in the Shiite holy city of Karbala and then shot them to death.

The U.S. military confirmed a report earlier Friday by The Associated Press that three of the soldiers were dead and one was mortally wounded with a gunshot to the head when they were found in a neighboring province, about 25 miles from the compound where they were captured. A fifth soldier was killed in the initial attack on the compound.

The new account contradicted a U.S. military statement on Jan. 20, the day of the raid on an Iraqi governor's office, that five soldiers were killed "repelling" the attack.

The security breakdown and the dramatic kidnapping and murder of four soldiers leaked out just as President Bush faces stiffening congressional opposition over his plan to flood Baghdad and surrounding regions with 21,500 more American troops. Two of Congress's most vocal war critics, House Speaker Nancy Pelosi and Rep. John Murtha, were in the Iraqi capital as the news broke.

In a statement issued late Friday, the military said two of the soldiers were handcuffed together in the back seat of an SUV near the southern Iraqi town of Mahawil. A third dead soldier was on the ground nearby. The fourth soldier died on the way to the hospital.

The brazen assault, 50 miles south of Baghdad, was conducted by nine to 12 gunmen posing as an American security team, the military confirmed. The attackers traveled in black GMC Suburban vehicles — the type used by U.S. government convoys — had American weapons, wore new U.S. military combat fatigues, and spoke English, according to two senior U.S. military officials as well as Iraqi officials.

The confirmation came after nearly a week of inquiries. The U.S. military in Baghdad initially did not respond to repeated requests for comment on reports that began emerging from Iraqi government and military officials on the abduction and a major breakdown in security at the Karbala site.

Within hours of the AP report that four of the five dead soldiers had been abducted and found dead or dying about 25 miles east of Karbala, the military issued a long account of what took place.

"The precision of the attack, the equipment used and the possible use of explosives to destroy the military vehicles in the compound suggests that the attack was well rehearsed prior to execution," said Lt. Col. Scott Bleichwehl, spokesman for Multi-National Division-Baghdad.

"The attackers went straight to where Americans were located in the provincial government facility, bypassing the Iraqi police in the compound," he said. "We are looking at all the evidence to determine who or what was responsible for the breakdown in security at the compound and the perpetration of the assault."

The Karbala raid, as explained by the Iraqi and American officials, began after nightfall on Jan. 20, while American military officers were meeting with their Iraqi counterparts on the main floor of the Provisional Joint Coordination Center in Karbala.

Three days after the killings, the U.S. military in Baghdad announced the arrest of four suspects in the attack and said they were detained on a tip from a Karbala resident. No further information was released about the suspects.

A senior Iraqi military official said the sophistication of the attack led him to believe it was the work of Iranian intelligence agents in conjunction with Iraq's Shiite Mahdi Army militia.

The Defense Department has released the names of troops killed Jan. 20 but clearly identified only one, Capt. Brian S. Freeman, 31, of Temecula, Calif., as being killed because of the sneak attack.

Also killed that day were four Army soldiers said to have been "ambushed" in Karbala.

The four were identified as 1st Lt. Jacob N. Fritz, 25, of Verdon, Neb.; Spc. Johnathan B. Chism, 22, of Prairieville, La.; Pfc. Shawn P. Falter, 25, of Homer, N.Y., and Pvt. Johnathon M. Millican, 20, of Trafford, Ala.