Image caption There have been several complicated issues with bringing the five defendants to trial

Lawyers for three men accused of plotting the 9/11 attacks want US President Barack Obama and predecessor George W Bush to give evidence.

The lawyers say the men cannot receive a fair trial after being regularly called "terrorists" and "murderers".

Ramzi Binalshibh, Mustafa Ahmad Al-Hawsawi, and Ali Abd Al-Aziz Ali will face a military trial at Guantanamo Bay and a possible death penalty.

The presidents are not expected to agree to give evidence.

The 11 September attacks killed 2,976 people in New York, Washington and Shanksville, Pennsylvania in 2001.

In a motion to dismiss the charges, the defence teams say the language used by the current and former presidents has exerted what is known in military law as "unlawful influence" on the case.

The BBC's Jonathan Blake, in Washington, says that the request highlights just one of several complicated issues that must be resolved before the case can begin and how difficult it has been for the US to bring people to justice over the 9/11 attacks.

'Objectivity impossible'

"Under these facts, it is impossible for any objective, disinterested observer, with knowledge of all the facts and circumstances, to believe these men can receive a fair trial by military commission," the lawyers wrote.

Who are the suspects? Khalid Sheikh Mohammed, the most high profile of the suspects, has allegedly admitted to masterminding the 9/11 attacks and others. Captured in 2003, he has been at Guantanamo Bay since 2006

Ramzi Binalshibh, a Yemeni who allegedly helped locate flights schools for the hijackers

Ali Abd al-Aziz Ali allegedly helped nine of the hijackers enter the US

Mustafa Ahmad al-Hawsawi, of Saudi Arabia, is said to have helped set up some of the hijackers with money, clothes and credit cards

Waleed bin Attash, a Yemeni, is said to have been a bodyguard to Osama Bin Laden and trained some 9/11 hijackers CIA waterboarding tapes revealed

"It can easily be understood by members of the public that this system of military commissions exists solely for the purpose of imposing a death sentence upon these accused."

The lawyers have also requested testimony from Vice President Joe Biden, Attorney General Eric Holder, South Carolina Republican Senator Lindsey Graham, who has been involved with detainee policy, as well as a number of Pentagon officials.

The motion was filed on 11 May but posted on a Pentagon website only on Wednesday after security clearance, is one of several pending pre-trial motions. Prosecutors have not responded to the motion.

In addition to the three men, Waleed bin Attash and Khalid Sheikh Mohammed, the self-described mastermind of the attacks, are also facing trial on charges of murder and terrorism.

They have not entered pleas on the charges.

In 2009, an effort to have them tried in a civilian court failed in the face of a public outcry and congressional opposition.

The five were eventually charged in June 2011 with offences similar to those they were accused of by the Bush administration.

One of the defendants' lawyers, James Connell, has predicted the trial would take years to complete.