A U.S. military statement issued Wednesday said the Marine helicopter was conducting "security and stabilization operations" when it went down. That description is used by the Marine Corps as a matter of policy in Iraq when it determines that specifying the circumstances surrounding casualties could aid the insurgents.
Abizaid cited severe weather, but said the cause of the crash was still under investigation. An Accuweather map showed sandstorms early Wednesday in western Iraq near the Jordanian border where the helicopter crashed, the Associated Press reported.
A Marine CH-53E Super Stallion helicopter similar to the one that crashed Wednesday is shown lifting an amphibious assault vehicle in western Iraq.
Maps and graphics illustrate the U.S. military losses in Iraq on Wednesday.
The day's U.S. death toll -- 34 Marines, two soldiers and one sailor -- was the worst of the war. On March 23, 2003, during the U.S.-led invasion, 28 U.S. troops were killed. Seventeen U.S. troops died on Nov. 15, 2003, when two Black Hawk helicopters crashed in Mosul in what the military said was likely the result of a rocket-propelled grenade attack.
A military statement Wednesday said four Marines assigned to the 1st Marine Expeditionary Force were killed while fighting insurgents in Anbar province. It provided no further details.
In the northern town of Duluiyah, insurgents attacked an Army patrol with rocket-propelled grenades Wednesday, killing one soldier from the 1st Infantry Division and injuring two others, said Maj. Neal O'Brien, a military spokesman.
In Baghdad, one soldier was killed and two were wounded by a roadside bomb, military authorities said. In a separate incident on Baghdad's airport road, seven soldiers were injured by roadside bombs.
The casualties on Wednesday brought the number of U.S. troops killed in Iraq to at least 1,414.
In the deadliest attack Wednesday targeting the election, a suicide bomber detonated a fuel tanker at the offices of the Kurdistan Democratic Party in the northern Iraqi town of Sinjar, killing five people and injuring at least 20, party officials told the Associated Press.
U.S. troops conducting raids in the nearby city of Mosul found the bodies of six Iraqis, Fox News Channel reported. Three car bombs exploded in Riyadh, about 15 miles west of Kirkuk, according to al-Jazeera television. One of the explosions killed two policemen, one Iraqi soldier and two civilians, the network said.
A message attributed to Zarqawi on the Internet warned Iraqis again against participating in the elections. "Here are the Americans calling for fake and fabricated elections," the statement said. Interim Prime Minister Ayad "Allawi's soldiers are competing to protect their brothers, the Jews and Christians," it said.
In Baqubah, about 35 miles northeast of Baghdad, insurgents attacked the offices of the Kurdish Democratic Party and the Iraqi National Unity Gathering party. They shot at security guards and set a bomb that later exploded, injuring one person, said Ihsan Kadhimi, a guard. The Associated Press reported that a traffic policeman was killed in the attack.
In one of several attacks against polling locations, a bomb exploded inside a school in the village of Nai, about 50 miles north of Baghdad. A guard, who refused to give his name, said, "The bomb exploded at 10:30 a.m. and it was terrible. The whole school is destroyed now."
Casey joined other U.S. and Iraqi officials in dismissing hints by Iraq's interim interior minister, Falah Naqib, that Zarqawi had been captured.
But one senior Iraqi official, speaking on condition of anonymity while discussing intelligence matters, said an individual in custody had met with Zarqawi two weeks ago. "We're talking about a very unstable Zarqawi," the official said. "He's not feeling good. He's lost a lot of senior people."
Iraqi officials announced additional security measures in an effort to provide security for the elections. Naqib told reporters: "I personally am optimistic. I think there will be some terrorist activities. But I don't think they will prevent people from voting."
Correspondent Steve Fainaru in Mosul, special correspondents Omar Fekeiki in Baghdad, Hassan Shammari in Baqubah, Saad Sarhan in Najaf and Salih Saif Aldin in Baiji, and staff writers Josh White and Bradley Graham and researcher Julie Tate in Washington contributed to this report.