The number of U.S. service members who have died since the Iraq conflict began March 20 reached 500 Saturday when a roadside bomb leveled a 4th Infantry Division (search) vehicle north of Baghdad, killing three U.S. soldiers and two Iraqi civil defense personnel.

Two Americans also were wounded in the attack, which occurred when a Bradley Fighting Vehicle (search) struck an explosive device on a road near Taji, about 20 miles north of Baghdad, said Lt. Col. Bill MacDonald, a spokesman for the 4th Infantry Division.

The vehicle caught fire, killing the three Americans and two Iraqis who were on joint U.S.-Iraqi patrol, MacDonald said.

A U.S. quick reaction force rushed to the area and detained three men fleeing in a white truck, he added. Soldiers found bomb-making material in the vehicle, he added.

Also Saturday, the military said a U.S. soldier died from a non-hostile gunshot wound south of Baghdad. The incident occurred Friday evening near Diwaniyah (search) south of Baghdad, the command said in a statement. No further details were released.

The commander of U.S. forces in Iraq has ordered a criminal investigation into reports of abuse of prisoners at a coalition detention center.

A military statement on Friday gave no indication about the scope of the alleged abuse, saying simply that Lt. Gen. Ricardo Sanchez ordered a probe "into reported incidents of detainee abuse at a coalition forces detention facility." The statement did not specify the facility.

"The release of specific information concerning the incidents could hinder the investigation, which is in its early stages," the statement said.

In Washington, Lawrence Di Rita, spokesman for Defense Secretary Donald H. Rumsfeld, said it is a criminal investigation and that the reports of abuse were deemed "very serious and credible."

Di Rita declined to provide any details other than to say the alleged abuse happened at detention centers in Baghdad.

The announcement followed allegations by Amnesty International and former prisoners of harsh treatment of detainees arrested by U.S. and coalition forces since the Iraq war began last March.

The coalition is believed to be holding about 12,800 detainees for various offenses, including attacks on U.S. and allied troops. Earlier this month, three U.S. Army reservists were discharged for abuse of prisoners at the Camp Bucca detention center in southern Iraq.

On the political front, Grand Ayatollah Ali al-Husseini al-Sistani, the country's most influential Shiite Muslim leader, demanded that members of a new provisional legislature be chosen by voters. The Americans want them selected by regional caucuses.

Doubts over the American plan for transferring power to Iraqi hands by July 1 have loomed over the U.S.-led occupation this week, with the Americans pointing to sporadic violence as evidence the country is not ready for direct elections.

U.S. officials insist al-Sistani's demand for elections is unfeasible given Iraq's security situation. Many Shiites suspect the Americans simply want to manipulate the caucuses to make sure favored Iraqis win seats.

L. Paul Bremer, the U.S. administrator for Iraq, said in Washington that the United States will revise its plan to create self-rule in Iraq, but he rejected postponement of a June 30 deadline for ending the occupation and handing over power.

"The Iraqi people are anxious to get sovereignty back, and we are not anxious to extend our period of occupation," Bremer said after meeting with Bush and senior U.S. officials.

Bremer, and an Iraqi delegation led by Adnan Pachachi, current chairman of the U.S.-appointed Iraqi Governing Council, plan to confer with Secretary-General Kofi Annan on Monday in New York.

A day after the demonstration, al-Sistani's representative in Basra sent a letter to Bush and British Prime Minister Tony Blair questioning the sincerity of the power transfer plan.

Shiites comprise about 60 percent of Iraq's 25 million people.

"Many political analysts are saying that the purpose of the hasty agreement ... was propaganda for your re-election campaign, especially after what your country has suffered because of military losses in Iraq," the letter by cleric Ali Abdul Hakim al-Safi said.

In other developments:

-- France, a leading opponent of the war, said it wants to help to train Iraq's next generation of police officers -- once power is transferred to a sovereign Iraqi government. But Foreign Minister Dominique de Villepin said Friday that the question of sending in French troops is "not a current topic."

-- Turkey's military warned Friday that "bloody" internal fighting could erupt in Iraq if political power there is divided up based on ethnicity. Turkey is worried about demands by Iraqi Kurds for greater autonomy in oil-rich northern Iraq, which it fears might bring instability to its own borders.

-- An advance team of Japanese soldiers arrived Saturday in Kuwait for training at a U.S. military base before they cross overland to Iraq on a humanitarian mission that puts soldiers from Japan in a combat zone for the first time since World War II.

Fox News' Dana Lewis and The Associated Press contributed to this report.