Two American soldiers have been killed and a half-dozen wounded in separate outbreaks of violence in Iraq. The latest casualties come as investigators into Tuesday's bomb attack on United Nations headquarters in Baghdad begin to examine whether Iraqi security guards were involved in the attack.

Military officials in Iraq say a U.S. marine was killed in an attack in the town of Hilla, 50 kilometers south of Baghdad. A spokesman said the marine was shot to death by an unidentified gunman while in a vehicle that was stopped by traffic congestion.

In addition, a U.S. Army soldier is reported to have died from smoke inhalation during a fire at a shooting range in the Iraqi capital. Six other soldiers were reportedly wounded. No details were provided on possible causes of the fire.

In a third episode, six U.S. soldiers were wounded Friday when their truck ran over a roadside bomb near the town of Baiji, 200 kilometers north of Baghdad. U.S. military officials say the wounded were evacuated to a field hospital at nearby Tikrit.

The violence brought to 64 the number of U.S. soldiers who have been killed since the major fighting in Iraq was declared over nearly four months ago.

U.S. officials blame the attacks on individuals loyal to the deposed regime of Saddam Hussein. But some senior officials say Hussein loyalists are possibly being assisted by foreign fighters who have infiltrated Iraq from neighboring countries.

Investigators into the deadly truck bombing at the United Nations headquarters in Baghdad reportedly have begun to focus on suspicions that some security guards at the headquarters may have provided information to the attackers.

A truck packed with explosives blew up a corner of the building, killing at least 22 people, including the U.N. senior representative to Iraq, Sergio Vieira de Mello.

Officials involved in the investigation reportedly have said the timing of the attack and the position of the truck bomb would indicate the attackers had inside knowledge of the whereabouts of Mr. Vieira de Mello at the time. Some of the guards, who had been hired many years before, are said to have worked for intelligence services of the Saddam Hussein government.

However, the head of the new Iraqi police force, General Ahmed Ibrahim, told reporters that it is too early to tell who was behind the U.N. bombing.

General Ibrahim said it could be remnants of the Saddam Hussein regime, or it could be what he called "criminals." He said more evidence is needed.

A previously unknown group, the Armed Vanguards of a Second Mohamed Army, has claimed responsibility.

The United Nations continues to evacuate the wounded as well as nonessential personnel in Iraq. The remains of those killed in the attack are being flown out of the country in the coming days.