“I cannot believe I am still alive,” Mr. Younis said.

For Iraqis, the pain of not knowing can be the worst of all. The International Commission on Missing Persons, a Netherlands-based organization, has estimated that up to a million Iraqis have gone missing in recent history. That encompasses the war between Iran and Iraq, the mass killings ordered by Mr. Hussein after a Shiite uprising in 1991, the Iraqi government’s Anfal chemical-weapon strikes against the Kurds in the late 1980s, and the more recent sectarian civil war of the last decade.

The commission noted on its website that there are “millions of relatives of the missing in Iraq who struggle with the uncertainty surrounding the fate of a loved one.”

Go anywhere in Iraq, especially in the south where Shiites dominate, and knock on almost any door, and you will hear a story of a lost loved one and, improbably, of a remaining shard of hope.

Nihad Jawad, a teacher from the southern city of Hilla, said that one night in 1991, her brother left home and was never heard from again. She has heard all sorts of rumors — that he was seen being apprehended by the military, that he was shot. “We searched everywhere for him, and we have found nothing,” she said. “We still have hope that he is still held in one of the secret prisons.”

The Islamic State’s brutality has written a new chapter in that dark history. The number of bodies has overwhelmed the capacity of the Iraqi government, and very few of them are ever identified by DNA testing.

In Diyala Province, where the Islamic State was once strong, a father who lost his son about two years ago said he scours jihadist websites for videos that might show his missing child. He rushes to the scene of every mass grave uncovered in the province.