Medical records show that a September 11 terror suspect sustained a head injury while being held and interrogated by the CIA that has triggered lasting health problems, his lawyer said Wednesday.

Ali Abd al-Aziz Ali, who is among five Guantanamo Bay prisoners charged with plotting the 2001 attacks on US soil, told medics at the US prison that he suffered auditory hallucinations, vision problems, memory loss and delusions caused by the injury, attorney James Connell told a pretrial hearing.

The hearing was called to determine whether the 9/11 defendants are able to appropriately report claims of mistreatment, a right guaranteed under the UN Convention against Torture, during the three years that followed their 2003 arrest.

They were held in the CIA's network of overseas prisons during that period and subjected to special interrogation techniques their lawyers call torture.

The newly released medical report, dated September 2006, described "mistreatment" that took place "several years earlier," Connell said. He did not confirm how the head injury happened.

Connell said the government took no action after Ali made his claim of mistreatment.

The detainee "reported mistreatment and nothing happened, no investigation. These reports just moldered away," Connell told the judge in the military commission tribunal where the five men face the death penalty if convicted of plotting the attacks on New York and Washington that left nearly 3,000 people dead.

Lawyers for the accused also complain about the military commission's highly restrictive rules for handling classified evidence.

"The irony is that Mr Al-Baluchi cannot describe his clinical history, how he got his injuries... because everything he says would be treated as classified," Connell said.

But prosecutor Clay Trivett stressed that "the accused is currently in control of the US government. He's been exposed to secrets of the US government."

"We were at war and we needed intelligence," Trivett added, referring to the so-called "war on terror."

"The US didn't choose to go to war but Al-Qaeda chose to go to war with us."

Only four of the accused -- Ali was absent -- attended the trial, wearing camouflage or traditional long white robes.

Ali, also known as Ammar al-Baluchi, is accused of helping organize logistics for the 9/11 attacks.

He is the nephew of the lead defendant and self-proclaimed mastermind of the attacks, Khaled Sheikh Mohammed, and the cousin of Ramzi Yousef, the convicted perpetrator of the 1993 World Trade Center bombing.

The 2012 film "Zero Dark Thirty" featured a character closely resembling Ali and identified as a detainee subjected to torture in a secret CIA prison.

Connell has recently asked to obtain the same information that the CIA provided to the film's producers but a federal US judge refused his request in a separate case.