The FBI has joined forces with Iraqi authorities in their investigation into the killing of an American contractor last month in Baghdad's Green Zone, FOX News confirms.

Iraqi forces detained five U.S. private security contractors in connection with the murder of colleague James Kitterman, 60, whose body was found May 22 in his car in the Green Zone. He had been blindfolded, bound and stabbed.

The five men — who have not been formally charged yet — could become the first Americans to face local justice since a bilateral security pact that came into force in January made U.S. contractors subject to Iraqi law.

"Embassy consular officials have visited the five and ensured they are being afforded their rights under Iraqi law. The men appeared well," a U.S. spokesman said Sunday.

The five contractors were detained early Friday morning when Iraqi authorities raided the contracting company's compound, Maj. Gen. Hussein Kamal, Deputy Minister of Interior, told FOX News.

Kamal, who declined to release the names of the detainees, said Iraqi forces also uncovered unregistered weapons and drugs during their raid of the compound.

An FBI official told FOX News on Sunday that the bureau is assisting with evidence collection and other investigative procedures. Iraqi forces asked the FBI for help, the official said.

An official of Corporate Training Unlimited, a Fayetteville, N.C.-based security company, told Associated Press that the five included Donald Feeney Jr., 55, who founded the company in 1986, his son Donald Feeney III, 31, and three other employees.

"I think everybody is devastated by the loss, including the Feeneys. And they're cooperating fully with the investigation," company spokeswoman Sarah Smith said. "They've not been charged with anything. And we suspect that they won't be charged with anything."

She said the CTU contractors knew Kitterman as "simply comrades living in the Green Zone."

"They ate meals together and just knew each other, I guess, being around one another. I think there was a mutual respect for one another and I know for a fact that the Feeneys are really devastated by his loss. The day he was murdered, I got a call saying how upset everyone was," she said.

Kitterman ran a small construction company based in the Green Zone and had worked in Iraq for about six years.

Iraq assumed control of the Green Zone on Jan. 1 under a U.S.-Iraqi security agreement, taking primary responsibility from the Americans for searching vehicles and checking identity papers as entry checkpoints.

Kitterman was believed to be the first American ever assassinated there since the protected area was established after the city fell to U.S. forces in April 2003.

Interior Minister Jawad al-Bolani, who supervises Iraqi police, said it appeared that Kitterman was killed because of an undisclosed "financial situation."

The Associated Press and Reuters contributed to this report.