The Swedish crime writer went to Africa to teach female guerrillas how to use grenade launchers, a friend has revealed

The life of Swedish crime writer Stieg Larsson reads like a thriller. The anti-fascist journalist – who died just before his bestselling Millennium trilogy was released – had his life threatened by far-right groups and could not marry his long-term partner for fear of compromising his safety.

Now another fascinating part of the author's life has been revealed: the year he spent training female guerrillas in Africa.

The information comes from John Henri Holmberg, a close friend of the writer. In a chapter of Afterword, a book sold as part of a boxed collection of Larsson's work, Holmberg describes how the writer taught Eritrean women to fire grenades. They were part of a Marxist liberation group fighting for the country's independence from Ethiopia, Holmberg explains.

Larsson's books have become a publishing sensation since his death in 2004: over 40m copies have been sold worldwide. Daniel Craig is filming the Hollywood version of the first in the trilogy, The Girl with the Dragon Tattoo, while the Swedish film of the The Girl Who Kicked the Hornet's Nest will open in the UK next month.

Despite worldwide interest in Larsson's colourful life, his revolutionary work was not known, at least in the UK. Holmberg gives few details of the period, merely stating: "1977 was a dramatic year. Stieg spent part of it in Eritrea, where he had contacts in the Marxist EPLF liberation movement and helped to train a company of women guerrillas in the use of grenade launchers. But he also contracted a kidney inflammation and was forced to leave the country."

Graeme Atkinson, the European editor of Searchlight magazine, yesterday confirmed Larsson had travelled to Eritrea to help in the independence movement. "Stieg was a revolutionary socialist and he believed in a better life, and equality for all," he said. "The fact there was crushing poverty in Africa appalled him."

Larsson put his life at risk during the struggle, according to the anti-racist campaigner, who described the Swede as one of his closest friends. "He went there to aid the struggle. That meant in the end being involved in fighting and he faced live bullets. He was an amazingly courageous man. He told me a lot about it, but never boasting. A lot of what he saw left me deeply shocked."

After his time in Africa, Larsson (who had undertaken military service in Sweden) joined his long-term partner, Eva Gabrielsson, in Stockholm, working for the post office. Later he was given a job by the country's largest news agency, TT, first as a graphic artist, then a journalist. He is said to have written his novels at night.

This week Larsson's father announced his son had been finishing a fourth book when he died from a heart attack at the age of 50. Larsson's partner and his family have been locked in a legal feud over control of the writer's estate.

Holmberg first met Larsson, whom he describes as a "card-carrying Trotskyite", at a science fiction convention, when Larsson was still a teenager. He later read the manuscripts for his novels. "I told Stieg they would make him rich," Holmberg wrote. "He said he knew that; they were his pension. I said he had no idea how rich ... At that he laughed."