The Pentagon has confirmed that five US citizens are being held in Iraq on suspicion of having links to insurgents or other criminal activity.
Cyrus Kar was found in a taxi with potential bomb components
The detainees have not been charged, and have not had access to a lawyer. A Pentagon spokesman declined to release their names.
However, one was identified by his family as Cyrus Kar, an Iranian-US film-maker and US navy veteran.
A US human rights group is suing the US government for his release.
The US spokesman, Bryan Whitman, said he had been found in a car which was also carrying several dozen washing machine timers - components that can be used in making bombs.
Officials said he was arrested with a cameraman and a taxi driver.
'Bring him home'
"He just had the misfortune to get into the wrong cab," said Steven R Shapiro, legal director of the American Civil Liberties Union.
"Our position is that if the government has any evidence against him, bring him home and charge in a court and then proceed accordingly."
Mr Kar's family in Los Angeles said the film-maker - who was born in Iran and moved to the US as a child - was in Iraq to film a documentary on King Cyrus the Great, who established Persia.
He was arrested in May.
Relatives of Cyrus Kar begged for his release
"I'm here to beg President Bush... to release an innocent boy," said 44-year-old Mr Kar's aunt, Parvin Modarress, at a news conference.
Three of the other detainees are of Iraqi descent, and one is originally from Jordan, the Pentagon said.
The Jordanian-American was captured late last year and is accused of having links with Abu Musab al-Zarqawi, the suspected terrorist leader and most wanted man in Iraq.
Of the Iraqi-Americans, one is alleged to have had knowledge of planning for an attack and a second may have been involved in a kidnapping, the US spokesman said.
The third was "engaged in suspicious activity", he said.
The authorities in Iraq were still deciding whether or not charges should be brought against the five, he said.
If so, it was unclear whether the cases would be handled by US courts or whether the five would go through the Iraqi legal system.