Lt. Col. Richard Williams, formerly chief of the Special Air Service, in an open letter to Prime Minister David Cameron, called for Sgt. Danny Nightingale's release, The Daily Telegraph reported. Nightingale was sentenced to 18 months last week after pleading guilty to the gun charge.
Williams commanded Nightingale in Iraq.
The letter was co-signed by Col. Tim Collins, former commander of the Royal Irish Regiment, and two former SAS men -- Andy McNab, author of "Bravo Two Zero," and Chris Ryan, who has written several books. They praised Nightingale's "17 years of exemplary service" in the military, including 11 with the SAS.
"We say this prosecution should never have happened. Furthermore, we say he was bullied into a guilty plea. And lastly, we say the custodial sentence is completely disproportionate to the alleged crime," the letter said. "We say he is the victim of a shameful travesty of justice and we demand immediate action."
Nightingale said the Glock pistol, found in a lockbox in his home, was given to him by Iraqi trainees. He said he had forgotten about the weapon.
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