Militants waging a campaign to derail Sunday's election carried out at least six car bombings and a flurry of other attacks on schools to be used as polling stations, political party offices and Kurdish sites, killing or wounding more than two dozen people.
While al Qaeda warned Iraqis to stay away from the polls — saying they would only have themselves to blame if they are hurt in attacks — and cast ballots in the crucial election.
A Bush administration official said the cause of Wednesday's crash was not immediately known but that there was bad weather at the time.
CBS News National Security Correspondent David Martin reports that the CH-53E "Super Stallion" helicopter went down early Wednesday morning near the town of Rutbah, 220 miles west of Baghdad in the al Anbar province.
The aircraft was transporting 1st Marine Division forces conducting "security operations," the U.S. military said in a statement. A search and rescue team has reached the site and an investigation into what caused the crash was under way.
An administration official said Wednesday the 31 people killed in the crash included 30 U.S. Marines and one sailor. The crash sets the record for the most American servicemembers to die in a single incident in Iraq.
It was also the deadliest day for U.S. forces since the March 2003 invasion.
In addition to Wednesday's crash deaths, four Marines were killed in fighting in Iraq's Anbar province, the military said. Also Wednesday, insurgents attacked a U.S. Army patrol near the northern town of Duluiyah, killing one soldier and wounding two others, and in the Baghdad area a roadside bomb killed another soldier and wounded two others, the U.S. command said.
The day's deaths brought to at least 1,409 the number of members of the U.S. military who have died in Iraq, according to an Associated Press count.
In other developments:
Mr. Bush expressed his condolences for the deaths. "The story today is going to be very discouraging to the American people. I understand that. It is the long-term objective that is vital — that is to spread freedom," he told reporters.
He said "a lot of Iraqis" were expected to participate in the elections. "Clearly, there are some who are intimidated," Bush said. "I urge people to vote. I urge people to defy these terrorists."
The previous single deadliest incident for U.S. troops was also a helicopter crash: In November 2004, two Black Hawk helicopters collided while trying to avoid ground fire, killing 17 servicemembers. Earlier that month, a Chinook transport helicopter was shot down by shoulder-fired anti-aircraft missile near Fallujah, killing 16 American soldiers and wounding 26.
The U.S. military has lost at least 33 helicopters since the start of the war, including at least 20 brought down by hostile fire, according to a Brookings Institution study.
Previously, the most Americans killed in one day came on March 23, 2003, when 26 troops were killed in various incidents during the U.S. military's drive to take Baghdad and topple Saddam Hussein. Bush declared major combat over on May 1, 2003, but fighting has continued.
With only days before the election, guerrillas carried out a string of attacks Wednesday targeting political groups and voting sites.
A suicide bomber detonated a vehicle packed with explosives at the offices of the Kurdistan Democratic Party in the town of Sinjar, just outside Mosul, killing or wounding at least 20 people, said KDP official Mahdi al-Harki.
Earlier in the day, gunmen opened fire with machine guns on the local headquarters of the Patriotic Union of Kurdistan and the Communist Party in the city of Baqouba, north of Baghdad, killing a traffic policeman. The KDP and PUK are the two largest Kurdish groups in Iraq and have formed a coalition along with other Kurdish groups to run in the election.
Insurgents set off three car bombs in rapid succession in the town of Riyadh, north of Baghdad, killing at least five people — including three policemen.
Al-Arabiya television broadcast a videotape showing three men identified by insurgents as election workers who were kidnapped in Mosul. The satellite station said the three were abducted by the Nineveh Mujahedeen, which threatened to attack polling stations on election day.
Iraqis will choose a 275-member National Assembly and regional legislatures. Sunni Muslim extremists have threatened to sabotage the election and many Sunni clerics have called for a boycott because of the presence of 170,000 U.S. and other foreign troops.