Guantánamo prison camp authorities tried to trick inmate Mohamedou Ould Slahi by forging a letter purportedly from his mother whom he had been unable to see for years, his brother Yahdih has said.

The ploy, which was intended to persuade him to cooperate with his interrogators, failed not only because they misspelt Slahi’s name but also because his mother could not write.

This week Slahi became the first inmate to publish a memoir while still incarcerated when Guantánamo Diary was published in 20 countries and serialised in the Guardian.

Speaking on Tuesday at an event organised by the Guardian in partnership with Canongate, the publisher of Guantánamo Diary, and PEN, the writers’ association, Yahdih Ould Slahi said his brother had not been able to see his mother before she died at their home in Mauritania in 2013.

Guantánamo Diary: ‘I smelled a letter that had touched my mom’ Read more

“They had been very close, the family debated whether to tell Mohamedou,” he said. “We didn’t want him to hear from the prison guards. At first we told Mohamedou that his mother was extremely ill.” Eventually, after receiving advice from officials of the International Committee of the Red Cross, the family wrote to Slahi to inform him of his mother’s death.

The 44-year-old engineer was first detained in 2001 in Mauritania at the request of the US government, then rendered to Jordan and Afghanistan and tortured, and then flown to Guantánamo.

He is one of two inmates whose “additional interrogation techniques” were personally approved by Donald Rumsfeld, then US defence secretary, according to a US Senate inquiry. Slahi was dressed in a burqa, deprived of sleep, subjected to strobe lights, doused in water, threatened with dogs, sexually assaulted by female interrogators and forced to bark and perform dog tricks.

He wrote his memoir by hand after learning English, his fourth language, from his Guantánamo guards and interrogators, and it was published this week after his lawyer, Nancy Hollander, battled for six years to have the document declassified.

Hollander told the event that Slahi’s descriptions of the abuse that he had suffered at Guantánamo had already been confirmed by both the Senate inquiry and a separate investigation by the FBI.

He had fallen under suspicion after fighting in Afghanistan in the early 1990s, but the only evidence linking him to al-Qaida at the time of the 2001 attacks was that which was extracted through torture.

Hollander said her client had been in a form of legal limbo since the US government lodged an appeal after a US district court judge ordered his release.

Larry Siems, a writer and former director of the Freedom to Write programme at PEN American Centre, told the event that he believed that in describing his relationship with his guards and interrogators, Slahi was “holding up a mirror” to the American people.

Extracts from the book were read at the event by the musician Brian Eno and the actor and director Peter Serafinowicz.

The American Civil Liberties Union has launched an online petition demanding Slahi’s release.