Woolwich murder suspect Michael Adebolajo was arrested by Kenyan authorities three years ago because they feared he was attempting to join an al Qaida-linked militant group, the country's anti-terrorism police said.
Adebolajo, 28, who remains in hospital following the killing of Drummer Lee Rigby, 25, was arrested with five other men near the border with Somalia, police chief Boniface Mwaniki told the Associated Press.
He said they believe Adebolajo was preparing to cross into the war-torn country to train and fight with militant group al-Shabab at the time. The confirmation of his activities in east Africa came as his brother-in-law claimed his family had proof he was tortured "violently and sexually" after he was arrested.
In an anonymous interview with ITV News he said the experience had changed him and accused the UK Government of leaving him at the mercy of Kenyan police.
"We (the family) contacted the British Government and essentially, they refused to do anything and the Kenyans were saying they were going to kill him, behead him," he said.
"We had clear proof that he was being tortured ...violently and sexually. It seemed almost as if the British government were not co-operating in any way with him or trying to help."
He added: "I would say he's always been different since then. You could almost say he's a changed man in certain ways. He was a lot quieter and quite bitter towards the fact that he wasn't getting any help from anyone." Mr Mwaniki told AP the claims of torture were untrue.
A Foreign Office spokeswoman confirmed that a British national was arrested in Kenya in 2010 and that it provided consular assistance.
A Kenyan government spokesman later said Adebolajo was arrested under a different name and taken to court before being handed to British authorities.
"Kenya's government arrested Michael Olemindis Ndemolajo. We handed him to British security agents in Kenya and he seems to have found his way to London and mutated to Michael Adebolajo," Muthui Kariuki said. "The Kenyan government cannot be held responsible for what happened to him after we handed him to British authorities."