A militia loyal to Iraqi Shi'ite cleric Moqtada al Sadr freed an American former soldier after holding him captive in Baghdad for nine months.
The American, identified as Randy Michaels, was shown on Iraqi television in a US military uniform with no insignia.
He was flanked by two members of parliament from Sadr's movement, including the parliament's first deputy speaker.
Mr Michaels was handed over to the United Nations mission in Baghdad , which said it was in touch with the US embassy.
The Sadrist politicians described him as an American soldier, but Michaels said he was a former service member working in a civilian capacity at the time he was captured, last June.
He said he had been held by the Yom al Maoud, or Promised Day Brigade, an offshoot of Sadr's Mehdi Army militia.
The Brigade remained armed after Sadr disbanded most Mehdi Army units in 2008.
"I was taken inside Baghdad and have been kept in and around different locations within the city by al Maoud," Mr Michaels said.
"It was explained to me that my release has been for humanitarian purposes and there was no exchange involved," he said in remarks shown on Iraq's Bagdadiya television.
Maha al Douri, a lawmaker from Sadr's bloc said: "We declare the release of the American soldier, Randy Michaels, without any compensation, according to the instructions of Moqtada al Sadr, as a gift from him to the soldier's family and to his people, and to correct the image of Islam."
Qusay al Souhail, deputy parliament speaker, said the leadership of the Promised Day Brigade had made the decision to free their captive in light of the confirmation that US troops had withdrawn from Iraq.
The UN mission in Iraq, UNAMI, said in a statement that Suhail and Douri had handed over "an American citizen whom they said has been in detention for about nine
months by an Iraqi group".
The statement continued: "UNAMI is currently in contact with the US Embassy in Baghdad to follow up on the matter. No further comments at this stage."
The US has not yet commented on the release.
The Pentagon has said none of its serving troops are believed to be held in Iraq since last month when it recovered the remains of the last missing soldier.
Nine years after the invasion that toppled Saddam Hussein, the US withdrew its forces from Iraq in December, with the exception of a few hundred service members stationed as part of the diplomatic mission at its embassy.
The US mission still includes 2,000 diplomats and, as of last year, 14,000 civilian contractors.
The embassy says the number of contractors has declined since then but does not release updated figures.
Sadr's Mehdi Army fighters controlled swathes of Baghdad and southern Iraq until they were largely defeated by Iraqi government forces and US troops in 2008.
Sadr disbanded most of the Mehdi Army and joined mainstream politics, and his followers are part of the governing power-sharing coalition.
Some Mehdi Army offshoots continued to battle US forces until they withdrew, and have claimed responsibility for kidnappings of foreigners. Those groups have mainly said they are disarming now that US troops have left.