Rossel’s father, a suit importer, was killed while packing the family’s belongings into cars to move out of Dora, an area in southern Baghdad controlled by Sunni Islamists. The Muhammads were never able to return, though a kindly neighbor drove their car to them in their new, mostly Shiite, neighborhood in Baghdad. They lost their past — photograph albums, diaries and heirlooms.

Not everyone in the family wanted to know what happened to the house, but Rossel was told that a Sunni family she did not know had moved in.

“I try to imagine my room and what they do in it,” she said, her voice intense.

Rasheed Hameed, a Sunni Kurd, was forced out of his house this summer in Baya, another southern neighborhood, and moved his family to safety in Syria. Back in Baghdad, he saved some of his furniture with the help of neighbors who have militia connections. His dresser, kitchen chest and bed frame stood awkwardly in a courtyard at his new house on Friday. Inside, several large printing machines sat like giant unwanted guests, the property of a previous owner.

“They destroyed all my life,” Mr. Hameed said, gesturing at the furniture. “For what? We don’t know. What is our crime?”

Early in the war, it was extremely rare that an Iraqi would know his or her attacker, but as time went on the violence moved closer to home. In the Karbala study, 47 percent of families said that their neighbors were directly or indirectly responsible for their flight. The men who tipped off the killer of Rossel’s father lived in the neighborhood and were working as movers for the family on the day he was shot.

Omar, an 18-year-old Sunni who withheld his family name for his safety, said that as Shiites took over his neighborhood in western Baghdad, childhoods spent together seemed never to have existed. Now he and his cousins change the subject when old Shiite friends walk past his stoop. Safe topics: electricity, girls and soccer.

“It’s true we used to play with them,” he said, “but we couldn’t read what was inside their hearts.”