A US military judge at Guantanamo Bay has ruled that one of the five alleged September 11 plotters, Yemeni Ramzi Binalshibh, should be tried separately.

Colonel James Pohl said it was first necessary to establish whether Binalshibh – alleged to have served as a liaison between the hijackers and al-Qaeda leaders – had the mental capacity to take part in the trial given a 2008 diagnosis by military doctors that he had a “serious mental disease,” according to Pohl’s order published by the Washington Post.

Col Pohl separated Binalshibh’s case from the others to avoid further delaying a trial that, 13 years after the September 11, 2001 airliner attacks, still hasn’t started.

Military prosecutor Mark Martins has expressed hope that jury selection could start in January 2015.

But according to lawyers involved in the matter, that is unlikely to happen before late next year.

In light of Col Pohl’s ruling, only four of the accused – including self-proclaimed Pakistani mastermind Khalid Sheikh Mohammed – will likely be present at the next preliminary hearings scheduled to take place at Guantanamo between August 11 to 15.

All could face the death penalty if convicted.

“The government is analysing the Military Commission severance order and remains on-track for previously scheduled hearings in August,” Myles Caggins, a Pentagon spokesman, said.

Late last year, Binalshibh was removed several times from a pre-trial hearing at Guantanamo after a series of outbursts and his lawyers have on several occasions denounced his alleged mistreatment while in detention at the US naval base in Cuba.

“Since November 2013, the Commission has suffered a series of delays in United States v. Ramzi bin al Shibh that have had a serious impact on the proceedings in the other four cases and calling to mind the concerns of the Commission two years ago,” Col Pohl wrote.

Binalshibh’s alleged crimes include helping the hijackers find flight schools in the United States and of financing the airliner plot that killed nearly 3000 people.

Meanwhile, a review board ruled on Friday that a Kuwaiti held at Guantanamo could be freed while another should remain behind bars.

Fawzi Al-Odah appeared at a hearing on July 14 and, by consensus, the Periodic Review Board “determined that continued law of war detention of the detainee does not remain necessary to protect against a continuing significant threat to the security of the United States”.

However, in the case of fellow Kuwaiti inmate Fayez Al Kandari, the board “considered that the detainee almost certainly retains an extremist mindset and had close ties with high-level al-Qaeda leaders in the past”.

His continued detention therefore “remains necessary to protect against a continuing significant threat to the security of the United States”.