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 say.
Confirmation that Michael Adebolajo was held in Kenya and deported to Britain will intensify calls for UK 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 UK government has confirmed that it provided Adebolajo with consular assistance when he was arrested.
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 Kenyan 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-Shabab," he told the Reuters news agency.
The group wants to impose a strict version of Islamic law across Somalia.
Spy agencies scrutinised
A UK foreign office spokesperson 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 Rigby. 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, Theresa May, UK home secretary, 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 news agency that both suspects were known to the MI5 domestic security service. However, neither was thought to pose a serious threat.
The British government also said it is forming a group to combat Muslim religious leaders and others whose words could encourage violence.
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 ultra-conservative religious leaders.
Tony Blair, as Britain's prime minister, had tried to tighten rules against preachers promoting violence 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 counterterrorism strategy.
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".