hirty U.S. Marines and a sailor were killed in a helicopter crash and six more troops died in insurgent attacks Wednesday in the deadliest day for U.S. forces in 15 years. Militants waging a campaign to derail Sunday’s election set off at least eight car bombings that killed 13 people and injured 40 others, including 11 Americans. The guerrillas also carried out a string of attacks nationwide against schools that will serve as polling centers. While al-Qaida warned Iraqis to stay away from the polls, saying they would have only themselves to blame if they were hurt in attacks, President Bush called on people to “defy the terrorists” and cast ballots in the crucial election. Crash kills all on board There were no survivors among the 30 Marines and one Navy corpsman in the CH-53 Sea Stallion helicopter that crashed near Ar Rutbah, about 220 miles west of Baghdad. A search and rescue team was at the site. All but three of the Marines had been based in Hawaii, according to Sen. Daniel Akaka, D-Hawaii. In three other incidents, four U.S. Marines were killed in action in western Iraq, a soldier was killed and two others were wounded when insurgents attacked an Army patrol near the northern town of Duluiyah, and a roadside bomb in the Baghdad area killed a soldier and wounded two others, the U.S. command said. The U.S. military has not experienced such a high loss of life in one day in 15 years, since an explosion ripped through a gun turret on the USS Iowa during a training exercise in the Caribbean in April 1989, killing 47 sailors. The helicopter crash occurred during severe weather, but its cause was still under investigation, said Army Gen. John Abizaid, chief of U.S. Central Command. An Accuweather satellite photograph of the region taken Wednesday showed a sandstorm in the area where the Sea Stallion went down, two days after more widespread sandstorms grounded all air traffic in Iraq. No sign helicopter was shot down Bush called the crash “very discouraging” in a news conference at the White House. The military gave no further details of the other incidents. Jim Dolan, a reporter for WABC-TV in New York who was embedded with the Marines in western Iraq, said the four Marines killed in the west died when insurgents ambushed a convoy leaving the town of Haditha, hitting a vehicle with a rocket-propelled grenade. Bombings, fighting continue News of the U.S. casualties came as Iraq was engulfed in a new wave of violence aimed at destabilizing the nation ahead of the election: A suicide bomber detonated a fuel tanker at the offices of the Kurdistan Democratic Party in Sinjar, southwest of Mosul, killing five people and injuring at least 20 others, KDP officials said. Gunmen opened fire with machine guns on the local headquarters of the Patriotic Union of Kurdistan and the Communist Party in Baqouba, north of Baghdad, killing a traffic policeman. The KDP and the PUK, the two largest Kurdish groups in Iraq, have formed a coalition along with other Kurdish groups to run in the election. Three car bombs exploded Wednesday in Riyadh, a tense town north of Baghdad, killing at least five people, three of them policemen. One of the car bombs targeted a U.S. convoy, but there was no report of casualties, police said. Four U.S. troops were injured in a car bombing Wednesday in Saddam Hussein’s hometown, Tikrit, the U.S. command said. Another car bomb targeted a multinational forces convoy on the road to Baghdad’s international airport, injuring four troops, the command said. In Baghdad’s Sadr City district, Iraqi forces backed by U.S. troops raided a Shiite mosque, detaining up to 25 followers of a radical cleric, Muqtada al-Sadr, police and the cleric’s supporters said. 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. Copter losses The U.S. military has lost at least 33 helicopters since the start of the Iraq conflict in March 2003, according to a study by the Brookings Institution, a Washington think tank. At least 20 were brought down by hostile fire, the institution said. Video: Increased violence feared Before Wednesday, the deadliest single incident involving U.S. troops in Iraq took place Nov. 15, 2003, when two Black Hawk helicopters crashed in Mosul after colliding while trying to avoid ground fire, killing 17 U.S. soldiers and wounding five. Earlier that same month, on Nov. 2, 2003, a Chinook transport helicopter was shot down by shoulder-fired anti-aircraft missile near Fallujah, killing 16 U.S. soldiers and wounding 26 others. Last month, a suicide bomb exploded at a mess tent in a base near Mosul, killing 22 people, including 14 U.S. soldiers and three U.S. contractors.