Their desperation adds to fears that Iraq's March 7 parliamentary election will fail to quell a Sunni insurgency by drawing former militants into the political process, and help heal the wounds of a sectarian war which has killed tens of thousands of Sunnis and Shi'ites since the 2003 U.S. invasion.
In the desert province of Anbar, families say they are paying thousands of dollars in blood money to prevent their sons from being executed when they are released from U.S. military detention. If they can't find the money, their sons often disappear -- sometimes back into the ranks of insurgents.
This has become a phenomenon in Anbar, said Ali Hammad, a prominent sheikh in Anbar who works on national reconciliation issues and mediates between warring tribes.
It is difficult for criminals to leave Iraq, and because they are rejected by the community and face tribal prosecution, they end up joining the same groups and killing again.
Iraq's Sunni heartland of Anbar was once an al Qaeda stronghold after its tribes sided with al Qaeda to fight U.S. forces. The tribes switched their allegiance to the U.S. military from 2007 after finding that the Sunni Islamist group was usurping too much power and imposing draconian intolerance.
Under a security pact that came into force last year, the U.S. military has been handing over to Iraq the thousands of people it detained during the war -- some for alleged links with Shi'ite militia and others for ties with the Sunni insurgency.
The Iraqi government has accused released detainees of being behind some devastating suicide assaults on public buildings in Baghdad since last August. The U.S. military says just 5 percent of the 90,000 detainees that have passed through its prisons since 2003 ended up being sent back for new crimes.
There is pressure in communities to join insurgents, the U.S. military said in a statement.
In Anbar, former police chief Major General Tareq Yusuf said police knew of at least 50 released detainees that had been killed by tribes since 2007.
We had no evidence but we knew that the tribes killed them, Yusuf said.
Often, police probably turn a blind eye. The dangers mean that not all of the 6,000 or so detainees still in U.S. custody are keen to be free.
Of course we want him to leave custody but we need to be sure that we will not lose him, said Abu Abdullah, a brother of a detainee held in Camp Cropper near Baghdad airport. Many tribes are eagerly awaiting his release to kill him.
Tribal traditions in Iraq often allow a tribe to pay blood money to compensate for a murder committed by a member.
In Anbar, that only works for those vaguely associated with al Qaeda. Tribal leaders have decided to reject blood money in the cases of those known to have killed as part of the group.
The fate of the sons of al Qaeda is death one way or another, said Sheikh Mohammed Khamees Abu Risha, who lost three cousins, four uncles, his father and a grandfather to al Qaeda.
Even when blood money is an acceptable penance, some ex-detainees have found it does not help their cause.
Abu Mustafa, a cousin of a former detainee who was released last year from the now-closed Camp Bucca detention center, said his relative was accused of killing a policeman in 2006.
The U.S. military released him for lack of evidence but even so we ended up paying 12 million dinars ($10,000) in blood money to the policeman's family after they threatened us, he said.
Then all of the families of policemen killed in the same neighborhood started asking us to pay blood money for their sons. It seems like our son killed all those people. We do not know where he is now and we cannot pay all this money.
Abu Abdullah said the pressure was pushing some to extremes.
My brother is one of hundreds, thousands who have no place to go except the armed groups. Where are they supposed to go?
(Additional reporting by Fadhel al-Badrani; Editing by Michael Christie and Angus MacSwan)