The bodies of 20 people - bound and shot in the head - have been found on a military firing range in the eastern suburbs of Baghdad.
A police source said the remains appeared to have been there for some time and their identities were unclear.
The bodies of the boys and men are now in a mosque in Baghdad.
Elsewhere, four American soldiers were killed by roadside bombs in two similar incidents west of Baghdad.
The US military said the attacks happened in the former insurgent stronghold of Fallujah.
French hostage freed in Iraq after five months
The French journalist, Florence Aubenas, and her Iraqi companion have been freed five months after they were taken hostage in the Iraqi capital, Baghdad.
The French President, Jacques Chirac, warmly welcomed the news, referring to the 'happiness of the entire nation' at the releases.
Mr Chirac added that both were in good health, and said Ms Aubenas would arrive back in France this evening.
Her Iraqi companion, Hussen Hanun al-Saadi, has been reunited with his family in Baghdad.
Ms Abuenas, who works for the French newspaper Libération, was abducted along with her companion in Baghdad on 5 January.
Little had been known about their fate since then, but insurgents released a video tape of her in March in which Ms Aubenas made an appeal for help.
The circumstances of her release have not been revealed, but the French Foreign Ministry has denied that a ransom was paid.
The hostage taking triggered a public campaign in France and Europe to have them released.
