One of the two men held on suspicion of killing a soldier Lee Rigby in Woolwich was arrested in Kenya in 2010, the Foreign Office has confirmed.
It said it gave consular assistance to Michael Adebolajo "as normal" in the circumstances.
He was believed to have been preparing to fight with Somali militant group al-Shabab, a Kenyan government spokesman told the BBC, and was later deported.
Police probing the murder have arrested a 22-year-old man in north London.
The arrest in the Highbury Grove area, on suspicion of conspiracy to murder, brings the total number currently being held in the case to six.
Members of Mr Rigby's family have visited the scene of the killing, laying flowers at Woolwich Barracks where the 25-year-old drummer with the Royal Regiment of Fusiliers was based.
They hugged and comforted each other as they looked at some of the many thousands of floral tributes that have been left in his memory, before crossing the road to look at the spot where he was attacked in the street.
Several hundred people gathered at the scene a few hours later, some chanting Mr Rigby's name and waving Help for Heroes flags.
Confirmation of Michael Adebolajo's
arrest in Kenya in 2010 - preparing,
according to the Kenyan authorities, to
train and fight in Somalia - raises
British security officials have had
long-standing concerns about the risk
of young men travelling to join the militant
group, al-Shabab, and returning to pose a
danger on the streets of the UK.
Earlier this month, when David Cameron
hosted a conference on Somalia he said
the challenges of terrorism and extremism
"matter to Britain - and to the whole
So you might have expected Michael
Adebolajo to have been firmly on the radar
of the security services when he returned to
the UK. They will now be under renewed
pressure over exactly what they knew about
him, and whether more could have been
done to prevent the killing of Drummer
The Met Police said it was a planned community event in which a group of people intended to lay a wreath.
A small group of English Defence League members also joined the crowd, prompting organisers to complain that their plans had been "hijacked".
The Kenyan government had previously denied that Mr Adebolajo had ever visited the country, but spokesman Muthui Kariuki said there had been some confusion as he was arrested under a different name.
In video footage of his court appearance which emerged on Sunday, Mr Adebolajo is heard to say: "These people are mistreating us, we are innocent."
Mr Kariuki told the BBC World Service's Focus on Africa programme that Mr Adebolajo was then handed over to "British security officers" when it emerged he was a UK citizen.
BBC News East Africa correspondent Gabriel Gatehouse says Mr Adebolajo was detained in late November 2010 on an island off the coast of Kenya, near the Somalia border.
But our correspondent says the Kenyan government does not believe he ever reached Somalia.
Islamist insurgent group al-Shabab is affiliated to al-Qaeda and is thought to have 7,000 to 9,000 fighters. It killed 76 people in a double bomb attack in Uganda as they watched the 2010 World Cup.
Mr Adebolajo, 28, and a second man, Michael Adebowale, 22, were arrested on suspicion of the murder of Drummer Rigby on 22 May.
They remain in custody in hospital in a stable condition after being shot and wounded by police at the scene after the killing.
Three further men, aged 21, 24 and 28, were arrested in London on Saturday evening on suspicion of conspiracy to commit murder - a Taser was used on two of them.
A 29-year-old man arrested earlier on suspicion of conspiracy to murder was released on bail on Saturday, while two women aged 29 and 31, arrested on Thursday, have been released without charge.
In an update on Sunday, Metropolitan Police Deputy Assistant Commissioner Stuart Osborne said officers were examining CCTV footage, social media and forensic material as part of their investigation into Drummer Rigby's murder.
He appealed for any associates of Mr Adebolajo and Mr Adebowale who believed they might have useful information to come forward.
'Refused to help'
Earlier, Home Secretary Theresa May told the BBC's Andrew Marr programme "500 officers and others" were working on the case, including counter-terrorism officers brought in from elsewhere in the country.
Senior Whitehall sources have previously confirmed to the BBC both suspects arrested at the scene of Drummer Rigby's killing were already known to security services.
In an interview with ITV News, Mr Adebolajo's brother-in-law, who was not named, said he was "tortured... violently and sexually" while in Kenyan detention, but when his family contacted the British government "they refused to do anything".
The brother-in-law also said Mr Adebolajo had been approached on his return to the UK by MI5 who "asked him would he be a spy for them", adding: "They basically pestered him for years, when he was trying to recover from something psychologically damaging."
The claims are similar to those made by a friend of Mr Adebolajo, Abu Nusaybah, to the BBC's Newsnight on Friday.
However, the Kenyan government spokesman Muthui Kariuki denied that Mr Adebolajo had suffered any mistreatment while in custody.
When asked if there were mistakes made by the security services in dealing with this case, Mrs May said: "What we have is the right procedures which say when things like this happen we do need to look at whether there are any lessons to be learned."
She also said a new taskforce was being set up to look at whether new powers were needed to tackle extremism.
It will be chaired by the prime minister and include senior cabinet ministers and security chiefs.
In other developments:
Prayers were said on Sunday at a service dedicated to Drummer Rigby at St Peter the Apostle Roman Catholic Church, in Woolwich, at a service at the town's St Mary Magdalene Parish Church and in his local church in his home town of Middleton, Greater Manchester
French authorities are treating the stabbing of a soldier in a Paris suburb as terrorism and are investigating whether it was a copycat attack. Defence Minister Jean Yves Le Drian said Private First Class Cedric Cordier had been targeted by an unknown man because of his profession
Mrs May also said "thousands" of people were potentially at risk of radicalisation in the UK and the government had introduced a new programme to help those who could be sucked in
David Cameron is to set up a taskforce to "look again" at the government's strategy for dealing with extremism and radicalisation
Former Conservative leader Lord Howard said the Tories could form a pact with Labour to push through the controversial Communications Data Bill, despite Lib Dem objections. The bill would give police and security services access to details of all online communication in the UK and supporters say it would help root out would-be terrorists