Complaints were filed by the Nigerian, Daso Abibo, 51, and by Deanna Burdine, 65, a former Homeland Security employee who says she walked in on the assault.

Abibo was deported to Nigeria for working in the United States without a permit. His effort to seek asylum was unsuccessful.

After the alleged assault, Abibo wrote a letter from the Oklahoma County Jail in which he said he was attacked by five or six Homeland Security employees after telling an officer he wanted his attorney to review a deportation-related document before signing or placing his fingerprint on it.

"In a flash, five, maybe six officers attacked me," he wrote.

"My legs were in shackles," he stated. "One officer grabbed my neck from the back with his hand pressing it. Another officer was holding tight on my ears, twisting and pulling hard on them as if to pull them off my head ... One officer was twisting my left hand, while another was busy knocking on my Achilles heel's tendon. It was a nightmare. It was so painful, I asked them to shoot me ... dead so they could get what they want.

"This is lawlessness under the law," he complained. "I never fought back."

Burdine supported Abibo's account in letters to the FBI and Oklahoma City federal prosecutor's office.

Burdine was fired in April. She says her dismissal was retaliation for filing the complaint. Burdine is seeking her job back through arbitration.

Amnesty International has written a letter to the director of Homeland Security's Office of Civil Rights and Civil Liberties protesting the alleged assault and asking for an investigation.

"According to current and former DHS employees in Oklahoma City, ICE officers routinely abuse detainees there," Susan Benesch of Amnesty International said in the letter. "There seems to be a pattern and practice of abuse on the part of ICE officers in Oklahoma City."

FBI spokesman Gary Johnson confirmed that the agency is investigating the case but declined to discuss any findings, The Oklahoman reported in Sunday's editions.

Carl Rusnok, regional spokesman for Immigration and Customs Enforcement, denied there has been any widespread abuse of detainees.

"There is no abuse of detainees by any ICE officers," he said.

Immigration and Customs Enforcement's regional office in Dallas issued a written statement saying the alleged assault is being investigated by the department's inspector general. No comment will be made until the investigation is complete, officials said.