BAGHDAD — Gunmen in Baghdad's Shaab neighborhood stormed into a house not far from an Iraqi police checkpoint and killed 11 members of an Iraqi journalist's family, witnesses and journalism organizations reported Monday.

Iraqi police and U.S. military officials said they had no record of the killings. But family members confirmed that the killings took place on Sunday in a neighborhood controlled by the Mahdi Army militia. The militia is loyal to radical Shiite Muslim cleric Muqtada al-Sadr.

It was the third mass killing reported in Baghdad since Friday, underscoring the fragility of recent declines in violence. Car bombings on Friday and Sunday killed at least 22 people and injured 96 in the worst such attacks since September.

Meanwhile, the U.S. military said American troops killed three people, including a child, Monday when they fired on a vehicle that failed to stop at a roadblock north of Baghdad. The incident occurred after U.S. forces had attacked suspected al-Qaida in Iraq members near the town of Bayji, killing four, a military statement said.

During that action, a vehicle failed to stop when soldiers signaled and fired warning shots. The soldiers then opened fire, killing two men in the car. Soldiers discovered the wounded child when they searched the vehicle later, but the child died while being treated at a military hospital, the statement said.

Sunday's killings revived fears that Iraqi security forces are in league with Shiite militias to carry out attacks. A family member who wasn't at the house when the attack occurred said neighbors told him that the gunmen arrived in a Toyota Land Cruiser with no license plates and used explosives without drawing a response from an Iraqi police checkpoint nearby.

The killings came after the journalist, Dhia al-Kawaz, who edits a Web site from Amman, Jordan, that's frequently critical of militia groups, was warned to stop his work, said the family member, who asked to be identified only as Abu Mohammed. Kawaz was in Amman during the attack.

"As you've noticed, there is no one seeking an investigation, and no investigation has been opened," said Ibrahim al-Saraj, who heads the Association to Defend Iraqi Journalists' Rights in Iraq. Saraj said the killings were part of a campaign to attack journalists "to dim the news in this country and to oppress journalistic freedom in Iraq."

Journalists are frequent targets of violence in Iraq. At least 206 journalists and media assistants have been killed in Iraq since March 2003, according to the Paris-based advocacy group Reporters without Borders, which condemned the killings on Monday.

But Sunday's killings also provided a chilling look at why Baghdad remains a city governed by fear, despite declines in violence hailed by Iraqi and U.S. officials.