One of two men arrested over the murder of a British soldier in a London street was detained in Kenya in 2010 on suspicion of seeking to train with an al Qaeda-linked group in Somalia, Kenyan police said on Sunday.
Confirmation that Michael Adebolajo was held in Kenya and deported to London will intensify calls for Britain's spy agencies to explain what they knew about the suspect and whether they could have done more to prevent Lee Rigby's killing on Wednesday.
The British parliament's security committee will next week investigate the security services' actions in the run-up to a killing that has put pressure on Prime Minister David Cameron to take a harder line on radicals.
The Nairobi government initially said Adebolajo had never visited Kenya. But on Sunday, Boniface Mwaniki, head of Kenya's anti-terrorism police, said Adebolajo was arrested in November 2010 and deported to Britain.
"He was arrested with a group of five others trying to travel to Somalia to join militant group al Shabaab," he told Reuters.
The Islamist force, which is linked to al Qaeda, wants to impose a strict version of Islamic law across Somalia.
A Foreign Office spokeswoman in London confirmed the arrest and said consular officials had provided assistance.
Adebolajo, 28 and Michael Adebowale, 22, are under guard in hospital after being shot and arrested after the murder of the 25-year-old Afghan war veteran. They have not been charged.
Spy agencies have come under scrutiny after uncorroborated allegations by a friend of Adebolajo on Friday that intelligence officers tried to recruit him six months ago.
Asked whether the security services had contacted the men, Home Secretary (interior minister) Theresa May told the BBC: "Their job is about gathering intelligence. They do that from a variety of sources and they will do that in a variety of ways. And yes, they will approach individuals from time to time."
A source close to the investigation told Reuters this week that both suspects were known to the MI5 domestic security service. However, neither was thought to pose a serious threat.
The government also said it is forming a group to combat radical Muslim preachers and others whose words could encourage violence.
Prime Minister David Cameron's office said the group aimed to fight radicalism in schools and mosques, tighten checks on inflammatory internet material, and disrupt the "poisonous narrative" of hardline clerics.
Rigby's killing fuelled public anger about radical Islam. It has also raised questions over whether more could have done more to prevent the attack and put pressure on Cameron to tackle suspected militants more forcefully.
Witnesses said the soldier's killers shouted Islamist slogans during the attack. Bystanders filmed one of the suspects saying it was in revenge for Britain's involvement in the wars in Iraq and Afghanistan.
Successive British governments have wrestled with how to prevent people from becoming radicalized without alienating the wider population with draconian measures.
Former British prime minister Tony Blair tried to tighten rules against hate preachers after the London bombings in 2005 that killed 52 commuters. The measures stirred a long debate over how to balance free speech and civil rights with a strong counter-terrorism strategy.
Britain's two-party coalition government is divided over a planned new law that would allow police and spy agencies to monitor people's use of the internet and mobile phones.
The Muslim Council of Britain, a religious umbrella group, said new government measures risked "making our society less free, divided and suspicious of each other".