BAGHDAD, Iraq - U.S. authorities said Wednesday a young American who was beheaded by militants had been warned by the FBI to leave Iraq and was offered a plane ride to safety at a time when a new wave of violence spread across the country, making road travel extremely dangerous.

Mystery surrounded not only Nicholas Berg's disappearance but also why he had been held by Iraqi police for about two weeks and questioned by FBI agents three times. Berg's family disputed U.S. officials' claims that Berg was never in U.S. custody.

"The Iraqi police do not tell the FBI what to do, the FBI tells the Iraqi police what to do. Who do they think they're kidding?" Berg's father, Michael, told The Associated Press from his home in West Chester, Pa., a Philadelphia suburb.

Berg was last in contact with U.S. officials in Baghdad on April 10, and his body was found Saturday in Baghdad. Staff members at the $30-a-night Al-Fanar Hotel in Baghdad told the AP that Berg stayed there for several days until April 10.

Two e-mails sent by Berg to his family and friends show the 26-year-old telecommunications expert traveled widely and unguarded throughout Iraq - an unsafe practice rarely done by Westerners.

The FBI warned Berg shortly before his disappearance that Iraq was too volatile a place for unprotected American civilians but he turned down a State Department offer to fly him home, U.S. officials said Wednesday.

Michael Berg said his son refused a U.S. offer in early April to board an outbound charter jet because he believed travel to the airport was too dangerous. American soldiers refer to the airport highway as "RPG Alley" because of frequent attacks by insurgents firing rocket-propelled grenades.

According to the State Department, Berg told an American diplomat in Baghdad that he preferred to travel on his own to Kuwait.

"At that time, the U.S. consular officer extended an offer to assist Mr. Berg to depart Iraq by plane to Jordan," said State Department spokeswoman Kelly Shannon. "We'd already discussed that possibility with his family, and we mentioned that to him, obviously, when we talked to him on the 10th."

His family said Berg had already intended to leave the country on March 30 but that his detention prevented him from doing so.

Berg first worked in Iraq in December and January and returned in March. He was inspecting communications facilities, some of which were destroyed in the war or by looters.

During his time in Iraq, he struggled with the Arabic language and worked at night on a tower in Abu Ghraib, a site of repeated attacks on U.S. convoys and the location of the notorious prison where U.S. soldiers abused Iraqi inmates.

Michael Berg told the AP that Nicholas' paternal aunt, now dead, married an Iraqi man named Mudafer, who became close to Nicholas. In one of the e-mails, Nicholas Berg describes going to the northern city of Mosul, where he introduced himself to Mudafer's brother, identified as Moffak Mustaffa.

"We got along splendidly," Berg wrote. "We spent a few hours and I helped him establish an e-mail account."

Berg notes that "my presence ... made him more concerned (about his own safety and probably mine too) than I've been the entire time I've been here."

The young man was beheaded on a video posted Tuesday on a Web site. It bore the title "Abu Musab al-Zarqawi shown slaughtering an American," referring to an associate of Osama bin Laden believed behind a wave of suicide bombings in Iraq.

In Washington, Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said it was likely that al-Zarqawi himself was "the lead perpetrator." Al-Zarqawi, a Jordanian, is wanted in the killing of an American diplomat in Jordan in 2002 and is suspected of ordering many suicide bombings in Iraq.

U.S. spokesmen Dan Senor and Brig. Gen. Mark Kimmitt were quick to offer statements of condolence to his family and to draw attention to the barbarity of his death. Senor also said that "to my knowledge" Berg was not affiliated with any U.S. or coalition organization, nor was he ever in U.S. custody.

However, Senor said Iraqi police arrested Berg in Mosul on March 24 because local authorities believed he may have been involved in "suspicious activities."

Senor refused to say more, citing the sensitivity of the case. But he did confirm that the Americans were aware Berg was in custody.