A statement from the US Central Command said another soldier was wounded in the blast that hit their convoy at Habbaniyah, 60km west of capital Baghdad.
The soldiers were from the First Brigade Combat Team of Task Force All American.
The attack came on a day when Iraqi police detained six police officers suspected of involvement in Tuesday's killing of two US government employees in Karbala.
"They are from our police department. They are suspected of being involved. The case is under investigation, but they are innocent until proved guilty," Karbala police spokesman Rahman Al-Mussawi Diab told AFP.
US investigators were looking into whether two US nationals and their interpreter were killed by Iraqi police officers or impostors in a carefully plotted assassination.
The two employees of the occupation administration and their female interpreter were shot dead at a checkpoint by men in police uniform late on Tuesday night as they headed to their offices in Hilla, 100km south of Baghdad, from Karbala.
The attackers were allegedly
wearing Iraqi police uniforms
"If in fact they were bona fide police officers who were on the payroll of the Karbala police force, that is significant and the implications of that are significant," a senior US official told reporters on Thursday.
"Because the implications are so enormous we want to make sure that we are as close to 100% positive as we can be."
On Wednesday, a spokesman said the FBI had been asked to lead an investigation into the attack, which happened as the three drove back to Hilla from the city of Karbala, 45km to the south, where they had been visiting a newly-opened women's rights centre.
Hilla police chief Keis Hamser Abud told AFP he believed that men dressed in police uniform and driving a pick-up truck had carried out the killing and said that his officers had later detained six men, most of them wearing uniforms.
"We are now investigating whether they are real policemen or not," he said.
In a separate incident a US soldier was killed in a bomb blast on Wednesday in the Baquba region 50 km northeast of Baghdad in the second troop death in less than 24 hours.
Also on Wednesday, Major General Charles Swannack, charged with the restive al-Anbar province and due to leave in weeks, became the first US officer to publicly criticise the US failure to fully equip the new police and Iraqi Civil Defence Corps (ICDC).
US officials regularly claim the Iraqi police and ICDC are fully armed as they accelerate the process of handing back security responsibility.
But Swannack said bureaucratic delays pushed back the arrival of the equipment for months on end, leaving police and ICDC vulnerable to their enemies.
Swannack counselled the US Marines, who are taking over in Anbar province, to buy the equipment themselves rather than wait for the money to come through the $18.4 billion supplemental budget administered by occupation administrator Paul Bremer.
"I wouldn't have bet on it coming, because it never came on my watch," Swannack told reporters.
The money has not materialised due to bureaucratic technicalities, as well government investigations into the contract-awarding process. In the latest case, the Pentagon cancelled a contract on Friday for outfitting the New Iraqi Army.
US military spokesman Brigadier General Mark Kimmitt said Swannack's frustrations were keenly felt by commanders around Iraq, but said bureaucratic delays were a fact of life.
The stunning admission from US generals comes one week after Iraqi police complained the US-supervised interior ministry failed to meet their requests for additional equipment during the period before the suicide bombings on a Muslim holiday that left more than 170 people dead.