At least three Iraqis were killed and one child was injured after American soldiers stormed a tiny one-room house north of Baghdad and opened fire, U.S. and Iraqi officials said Tuesday.

Iraqi police, relatives and neighbors said a couple and their 19-year-old son were shot to death in their beds late Monday. But the U.S. military said soldiers came under fire and killed two suspected members of a terrorist cell in self-defense. It said it did not know who shot the woman or the child.

The U.S. military reported only three dead, but Iraqi police said two young girls were wounded and one died Tuesday at a hospital.

It was the second time in as many days that the U.S. military conceded involvement in the death of Iraqi civilians.

On Monday, the military said it had accidentally killed nine Iraqi civilians, including a child, in an airstrike targeting Al Qaeda in Iraq south of Baghdad.

In both cases, the military acknowledged involvement in the killings only in response to media inquiries.

Both incidents raised fresh concerns about the military's ability to distinguish friend from foe — and to protect civilians in the line of fire — in its stepped-up campaign to uproot insurgents from Sunni areas around Baghdad.

The latest deaths occurred in the village of Adwar, 10 miles south of Tikrit. The predominantly Sunni area is home to many former members of Saddam Hussein's regime, and has been the frequent site of U.S. raids against Sunni militants.

Meanwhile, U.S.-backed tribesmen and police said they discovered about 50 bodies Tuesday in a mass grave near Lake Tharthar, an area northwest of Baghdad where hundreds of bodies have been unearthed in recent months. Al Qaeda in Iraq controlled the area, as well as huge swaths of Iraq's western deserts, until being ousted early this year in an uprising by local tribes.

The U.S. military confirmed the Adwar raid in an e-mail to The Associated Press, saying its troops came under small arms fire while entering the building, and that soldiers shot dead two men inside. A woman was killed and one child was injured, but it was unclear who shot them, the military said.

It said the nighttime raid was based on intelligence gleaned from an informant — opening the possibility that the military was misled into targeting the family, perhaps out of local Iraqis' tribal or sectarian motives.

The incident remains under investigation, the military said.

A cousin of the victims, Kareem Talea Hamad, 20, said he watched the killings from his house across the street, and gave a different account of events than the American military's version.

Hamad said U.S. soldiers opened the door to the small brick house and immediately opened fire, killing its unarmed residents: father Ali Hamad Shihab, 55, his wife Naeimah Ali Sulaiman, 40, and their son Diaa Ali, who was a member of a U.S.-backed neighborhood watch group.

Such groups, composed mainly of Sunni fighters partnering with the U.S. to oust Al Qaeda from their hometowns, have been targeted by other militants because of their alliance with U.S. and Iraqi forces.

The head of Adwar's Awakening Council, Col. Mutasim Ahmed, confirmed that Diaa Ali was killed. He also offered an explanation for the discrepancy between the U.S. military's account of what happened, and that of Iraqi police and witnesses.

"It seems that some gunmen were positioned near the house and they opened fire on the Americans who returned fire," Ahmed said.

Two other daughters were wounded and transported to hospitals, and one died Tuesday morning, Hamad, the cousin, said. An Iraqi police officer, speaking on customary condition of anonymity, confirmed Hamad's account.

A surviving daughter, Nawal Ali, 16, said she was inside the house at the time of the raid, and that an Iraqi interpreter working for U.S. forces tried to stop the American soldiers from killing her parents.

The unidentified interpreter rushed into the house after he heard gunshots, Ali said. "He shouted at the Americans, saying `What the heck are you are doing?"' she said.

"Then he pushed them away after they killed my family," Ali said. She credited the interpreter for saving the lives of two of her younger siblings, 5-year-old Hamzah and 6-year-old Asmaa.

Witnesses who went to the family's house early Tuesday saw three dead bodies, laid out in their blood-soaked beds. Bullet casings littered the ground.

Relatives and neighbors gathered at the house to mourn the family, and loudspeakers at a nearby mosque announced plans for a funeral.

Later Tuesday, the U.S. military issued a statement saying it "regrets the loss of an innocent civilian and the wounding of a child." It did not name the father and son, but claimed U.S. soldiers killed the men in self-defense.

In Taji, north of Baghdad, a homicide bomber detonated his explosives Tuesday near the convoy of a sheik working with U.S. forces, killing two of his followers, police said. Those killed were members of the Taji Awakening Council, a group of Sunni tribesmen north of Baghdad who have partnered with the Americans to oust militants from their hometowns.

The homicide attacker was standing near a cluster of shops waiting for Sheik Sahthir al-Khlifawi's convoy, when awakening council members spotted him, a police officer said on condition of anonymity because he was not authorized to speak to media.

The men approached him after spotting wires dangling from his jacket, and the man then exploded himself, the officer said.

Al-Khlifawi said one of those killed was his nephew.

"We have been expecting such terrorist attacks after we received several threats. I gave orders to intensify security measures in the area," the sheik said.

Separately, the U.S. military said it detained eight suspected militants Tuesday in operations to disrupt Al Qaeda in Iraq across northern parts of the country.