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BAGHDAD, Iraq (CNN) -- A bomb exploded Thursday at a soccer match in Falluja, killing eight people and wounding 13, a police official said.
It was the third attack this week at soccer matches in Iraq, according to Al-Jazeera.
Flanked by spectators, young men from the city's northeastern Juraishi neighborhood had gathered for the match when the blast occurred, the official said.
Thirty miles (48 kilometers) to the east in the capital, explosions killed 15 people at two other soccer games Wednesday, Al-Jazeera reported.
At least 12 people were killed when "two hidden bombs" exploded on a Baghdad field during a game, and three were killed outside Baghdad when two mortar shells landed during a game, the network reported, citing government and police officials
Police in Baghdad found four unidentifiable, bullet-riddled bodies Friday morning in two neighborhoods. (Watch how the death toll in Iraq keeps climbing -- 2:22)
The bodies showed signs of torture, a signature of the sectarian killings that have wracked Iraq since February's bombing of the Al-Askariya Mosque in Samarra. That attack inflamed already smoldering Shiite-Sunni hostilities.
Since Tuesday, more than 100 bodies -- most bearing gunshot wounds and apparently tortured -- have been dumped across Baghdad. Iraqi police discovered 49 bodies Thursday after finding four Wednesday and 60 on Tuesday.
Lt. Gen. Peter Chiarelli, the commander of the Multi-National Corps-Iraq, said Friday that the Baghdad security plan has made the city safer. Chiarelli has visited all of the areas now under coalition control and has seen people in the streets and at the markets, evidence they are returning to their neighborhoods, he said.
He acknowledged the problem of sectarian violence in the capital and said the military had to get a "handle" on how and why it was happening.
Al Qaeda is purposely exacerbating the problem, he said, by perpetrating "spectacular attacks" aimed at maximizing casualties and thereby inciting more sectarian violence. Their task has become easier in the "post-Samarra era," he said.
When a bomb goes off in a primarily Shiite neighborhood, there is a "reaction from death squads, death squads who may move to Sunni neighborhoods," he said. Chiarelli added that similar retaliation occurs when an attack takes place in a Sunni neighborhood.
"And what we see in this instance is executions, executions of individuals, individuals that are picked off the streets sometimes from lists, taken to a location and tortured -- some not even taken to a location and tortured, and just executed," he said.
The U.S. military is working to rein in the violence, and "we're very, very pleased with what has occurred with the Baghdad security plan. And we look forward in the months ahead to seeing conditions in Baghdad continue to improve," Chiarelli said.
The U.S. military also suffered casualties when three U.S. troops died in separate operations, the military said Friday.
A U.S. Marine assigned to Regimental Combat Team 7 died Friday from "enemy action" in Anbar province, a Sunni stronghold west of Baghdad, the military said.
A roadside bomb exploded Thursday near a patrol, killing a U.S. soldier in northwest Baghdad, the military said.
And a Multi-National Division soldier based in Baghdad was killed when his vehicle was hit by a roadside bomb south of the capital. No other details were released.
In addition, a U.S. soldier remains unaccounted for after a deadly suicide truck bomb attack Thursday on an Army outpost in Baghdad, a U.S. military commander said Friday. (Watch as wounded troops are rushed into a combat hospital -- 1:14)
The bombing killed two soldiers and wounded 30 others.
"I'm sorry to add we now have a soldier that we are counting as missing," Chiarelli told reporters. The military listed the soldier's status as "Duty Status Whereabouts Unknown."
The attack appeared to hit troops while they were resting at the outpost. Some were wearing sneakers instead of combat boots when they arrived at the hospital. The division's commander, Maj. Gen. J.D. Thurman, visited the wounded shortly after the attack.
One soldier was killed at the scene, while the second died of his wounds at a hospital, doctors at a U.S. combat hospital said. One of the troops was classified as "very seriously injured," while another was "seriously injured." Eleven have returned to duty.
The number of U.S. troops killed in the Iraq war stands at 2,673. Seven American civilian contractors of the military also have died in the conflict.
More than 20,000 Americans have been wounded since the March 2003 invasion that toppled Saddam Hussein.
CNN's Mohammed Tawfeeq contributed to this report.