BAGHDAD — In mounting bloodshed south of Baghdad, suspected Shiite Muslim militiamen stormed into a Sunni Arab home yesterday and gunned down seven family members, including a baby being bounced on her mother’s shoulder.

Shadowed by the violence, American ambassador Ryan Crocker, co-author of a highly anticipated report to Congress next month, said Washington’s blueprint for reconciliation was insufficient to win back control of Iraq. Congressional benchmarks don’t tell the whole story, he said.

Mr. Crocker and the American military commander, General David Petraeus, may be heading into a storm of discontent as they argue before Congress that American troops need more time in Iraq.

The brutal attack south of Baghdad killed seven members of 70-year-old Khayrallah Salman’s family. He ran a small grocery in Mahaweel, 35 miles south of Baghdad, and died along with six relatives, including the 6-month-old girl, a 12-year-old girl, and two women. A son and daughter-in-law were wounded, Captain Muthanna Khalid of the Babil province police said.

A witness said the baby’s mother, who survived, was bouncing the child on her shoulder when the gunmen opened fire after breaking into the house about 8:30 a.m. The witness would not allow use of her name, fearing retribution.

Other witnesses and neighbors said Shiite Mahdi Army militiamen were responsible for the killings of the family, members of the Sunni al-Janabi tribe.

Police did not give a motive, and allegations against the militia could not be independently confirmed.

In other Iraq news, Saddam Hussein’s cousin, known as “Chemical Ali,” and 14 others went on trial yesterday on charges of crimes against humanity in the brutal suppression of a Shiite uprising that killed tens of thousands after the 1991 Gulf War.