Mortar shells rained down Sunday on a girls' secondary school in a mostly Sunni area of western Baghdad, killing five pupils and wounding 20, witnesses and police said. At least seven other people died in a series of bombings and shootings across the capital, mostly in Shiite areas. Elsewhere, Iraqi troops backed by US helicopters battled insurgents Sunday 20 kilometers (12 miles) northeast of the Shiite holy city of Najaf, Iraqi officials said. Provincial Gov. Assad Sultan Abu Klil said a US helicopter went down during the fighting, but US officials would not confirm the report. Llil said the operation was launched after the government received reports that insurgents planned to assassinate Shiite clerics and pilgrims during the Ashoura festival, which reaches its climax Tuesday. Also Sunday, US troops captured 21 suspected terrorists including an al-Qaida courier in a series of raids in Baghdad and Sunni areas north and west of the capital, the US command said. Three of the suspects were believed to have close ties to the leadership of al-Qaida in Iraq, the military said. The US military also reported Sunday that an American Marine died the day before from wounds suffered in fighting in Anbar province, a stronghold of Sunni insurgents. The Marine's name was withheld until his family is notified. The mortar attack occurred about 11 a.m. at the Kholoud Secondary School in the Adil neighborhood of western Baghdad. Several projectiles exploded in the courtyard of the school, shattering windows in the classrooms and spraying pupils with shards of glass. Pools of blood smeared the stone steps and walkways. Hours after the attack, grieving parents wept as the bodies of the victims were placed inside wooden coffins. Police said four girls were killed instantly and a fifth died later of her wounds. It was unclear who fired the mortars, but the area has been the scene of reprisal attacks by Sunni and Shiite extremists that have persisted as US and Iraqi soldiers prepare for a massive security crackdown. A Sunni group, the General Conference of the People of Iraq, accused Shiite militias and said in a statement that the markings on the mortars indicated they were manufactured in Iran. More than 150 people, mostly Shiites, have been slain in bomb attacks in the last week as the majority Islamic sect in Iraq celebrates a 10-day festival leading up to Ashoura, the holiest date in the Shiite calendar. Elsewhere, a bomb exploded about 7:30 a.m. in a minibus carrying passengers to a predominantly Shiite neighborhood in Baghdad on Sunday, killing one and wounding five, police said. The explosive device was hidden in a bag left by a passenger who got off the bus before it detonated in the Baladiyat neighborhood in eastern Baghdad. The bus was heading to the adjacent Shiite district of Sadr City, which has been targeted several times in the past. A parked car bomb exploded in an intersection near an outdoor market in the Sadr City district about five hours later, killing at least four people, two of them women, and wounding 39, police said. The sprawling Shiite slum is a stronghold of the Mahdi Army that is loyal to radical cleric Muqtada al-Sadr and has blamed for much of the country's spiraling violence. About five minutes later, a bomb hidden in a bag exploded in an outdoor market in the Baiyaa neighborhood in western Baghdad, an area that is mostly Shiite, although a significant number of Sunnis live there. At least two people were killed and 17 were wounded, including two children, police said. Outside the capital, a car bomb exploded near a mosque in the Sunni city of Fallujah, 65 kilometers (40 miles) west of Baghdad, killing two civilians and wounding four, police said. Iran, meanwhile, closed several border crossings with Iraq for Ashoura, which culminates on Tuesday with processions and ceremonies, including self flagellation, to mark the Shiite saint Imam Hussein's death in a battle. Iranian state television said the crossings were closed to "contain the large number of pilgrims" bound for the Shiite holy city of Karbala in southern Iraq who were planning to cross into Iraq without "legal documents." No other details were given but the report indicated that not all border crossings had been closed and that some pilgrims were allowed through elsewhere. Also Sunday, drive-by shooters killed a high-ranking Shiite official at the Iraqi industry and mines ministry, along with his 27-year-old daughter and two other people. Insurgents have frequently targeted high-ranking Iraqi officials who are seen as collaborators with the US forces. Last Wednesday, Iraq's higher education minister, a Sunni, escaped an assassination attempt after gunmen opened fire on his motorcade as he was traveling in southern Baghdad, killing one of his guards and seriously wounding another.