GUANTANAMO BAY U.S. NAVAL BASE, Cuba (Reuters) - Accused September 11 mastermind Khalid Sheikh Mohammed has been tinting his beard red by rubbing it with fruit juice and crushed berries from his breakfast, a Pentagon spokesman said on Tuesday.

Mohammed first showed up for his April arraignment hearing with his long, scraggly beard tinted a rusty red, and it remained dyed the same hue when he returned last week to the courtroom at the Guantanamo Bay U.S. Naval Base in Cuba.

That sparked a flurry of questions, since the rule book for the detention operation known as Joint Task Force Guantanamo, or JTF-GTMO, specifically prohibits prisoners from receiving hair dye because it can contain chemicals such as ammonia that could be used as a weapon.

Journalists asked whether someone had been smuggling contraband henna to Mohammed, who is held at a top-security camp whose very location on the Guantanamo base is kept secret. Henna is a plant often used to make hair dye.

A Pentagon spokesman, Army Lieutenant Colonel Todd Breasseale, revealed the answer on Tuesday.

"I can confirm that Mr. Mohammed did not avail himself of any outside-the-JTF means to dye his beard but did craft his own natural means by which to do it," Breasseale said, explaining the inmate used fruit juice and berries from breakfast.

He said he did not know Mohammed's reasoning.

It is not uncommon for men in the Muslim world to dye their beards with henna, as the Prophet Mohammad is said to have done.

Or it could be a case of simple vanity. At age 47, the alleged architect of the hijacked plane attacks that killed 2,976 people has gone gray in the beard.