WASHINGTON – Five detainees who are believed to be American citizens are being held in American military detention facilities in Iraq following their arrests there over the past few months. They are the first Americans taken into custody during the war in Iraq on suspicion of aiding the insurgency or for terrorist activity, Pentagon officials said yesterday.

In addition to one detainee with dual American-Jordanian citizenship who was arrested in late October, coalition forces have snared four suspects since April in unrelated cases involving potential insurgent activities throughout Iraq, a Pentagon spokesman, Bryan Whitman, said. Three of those arrested are Iraqi-Americans and one is an Iranian-American who claims he was in Iraq to film footage for a historical documentary.

All of those arrested held dual citizenship, but defense officials would not identify the detainees by name or divulge where they lived in America. It was also unclear yesterday how involved the detainees were in the actual fight against the coalition.

Mr. Whitman said one of the Iraqi-Americans was arrested for “engaging in suspicious activities,” another for alleged involvement in a kidnapping, and the third for “having the knowledge of planning associated with attacks on coalition forces.”

The Jordanian-American, arrested after a search of his Baghdad home in late October, is believed to be a high-ranking associate of Abu Musab al-Zarqawi’s terrorist network. Officials described him as an emissary with intimate knowledge of and participation in terrorist activities in Iraq.

But in the case of the Iranian-American – 44-year-old Cyrus Kar of Los Angeles – lawyers who are working to return him to his home in America argue that he was swept up by mistake as he was traveling through Iraq in a taxi while working on a film documentary about an ancient Persian king, Cyrus the Great. A native Iranian who served three years in the U.S. Navy, Mr. Kar was arrested by Iraqi Security Forces almost immediately after he entered Iraq from Iran on May 17, when soldiers found several washing machine timers in the taxi’s trunk.

Those timers can be used on improvised bombs, military officials said, and are a trademark of insurgents who have launched attacks on coalition forces throughout the country. The soldiers took Mr. Kar, his Iranian cameraman, and the taxi driver into custody, and Mr. Kar eventually landed at Camp Cropper, the highest-level American detention facility in Iraq.

Mr. Kar’s story, which plays out in a legal petition filed in federal court in the District of Columbia, was first reported by the New York Times and the Los Angeles Times. In the court papers, Mr. Kar’s family members claim that he has been held without reason for nearly two months and that the FBI has cleared him of wrongdoing after searching his home and files and after he apparently passed a lie detector test. But he remains behind bars in Iraq, without significant contact with the outside world and with no charges filed against him.

“We don’t understand why they won’t let him come home, especially since the government said he hasn’t done anything wrong,” Mr. Kar’s first cousin, Shahrzad Folger, said in a statement released yesterday.

A spokeswoman for the FBI field office in Los Angeles, Kathy Viray, said she was prohibited from providing any details about the Kar case. An American law enforcement official in Washington confirmed that Mr. Kar’s home was searched by the FBI, as alleged in the lawsuit, but declined to provide further details.

Lawyers with the American Civil Liberties Union filed the petition yesterday, claiming that Mr. Kar is being held by the American military “without the slightest hint of legal authority.” A Pentagon spokesman who specializes in detention operations, Lieutenant Colonel John Skinner, said yesterday that all of the American citizen detainees have been treated in accordance with the Geneva Conventions and have been treated humanely. All five of the detainees are being held at one of three main detention facilities – Camp Cropper near the Baghdad International Airport; at the Abu Ghraib prison west of Baghdad, or at Camp Bucca, near the southeastern port city of Umm Qasr.

Colonel Skinner said nationality is nearly irrelevant when people are taken into custody on suspicion of wrongdoing.

Pentagon officials said there are approximately 420 foreign nationals in American detention facilities in Iraq, many of whom are Syrian, Saudi Arabian, and Iranian, among many others. The American nationals make up a tiny fraction of the 10,000 detainees currently in custody, and an even smaller percentage of the more than 70,000 detainees who have been held in Iraq and Afghanistan since the wars began.

“If you’re engaged in suspicious activities or acts, or with suspicious individuals, you’re going to be scrutinized heavily, and nationality doesn’t play a role in that,” Colonel Skinner said.

But the American citizens’ detention in American military facilities raises distinct legal issues because the Supreme Court has ruled that even enemy combatants who are American citizens deserve certain rights – such as a legal status hearing and access to a lawyer – rights the detainees have not yet been able to exercise.