BAGHDAD, March 29, 2006 — -- The al-Ubaiedi family is getting used to its new house. They are now refugees in their own country.
She's a Shiite, he's a Sunni. For that, they received a threatening letter warning them to leave their Shiite neighborhood.
"It was better to move," said Farouk al-Ubaiedi, "than to have one of my children killed."
Across Iraq, families in mixed neighborhoods have received anonymous threats. One letter reads, "You have three days to leave or you will be killed."
"You have 48 hours," said another, "or your house will be turned to ash."
"Some of them must leave behind even their identification," said Dana Graber, at the United Nations-affiliated International Organization for Migration for Iraq. "All of their food, their money."
Today Shiites are moving into a newly erected tent city in Najaf. "Why is this happening to us?" one woman wondered aloud.
In Baghdad, Shiites just fled a Sunni neighborhood to live among their own.
"There was killing there," one boy said. "We were afraid."
For the past three years in Iraq, Sunnis and Shiites have gone out of their way to downplay their differences. Both groups are Muslim. Generally speaking, they look alike. They have intermarried, and even entire tribes have mixed.
But the bombing of a holy Shiite shrine last month inflamed tensions beneath the surface. There have been hundreds of executions, and now, perhaps just as ominous, a fear-fueled migration.
"This is the beginning of the nightmare," said Fawaz Gerges, a professor at Sarah Lawrence College.
To Gerges, it resembles the beginning of Bosnia's ethnic cleansing or the beginnings of the civil war in Lebanon, which he lived through.
"My instincts tell me that we are at the early stages of what happened in Lebanon and other civil war contexts," he said.
The al-Ubaiedis said they pray there will be no civil war and that they can someday go home.
ABC News' Dan Harris reported this story for "World News Tonight."