One of the men arrested over the murder of a soldier in Woolwich was also arrested in Kenya in 2010, anti-terror police in Nairobi have confirmed.
Michael Adebolajo appeared in court in Mombasa suspected of leading a group of Islamists trying to join the al Shabaab group.
Anti-terrorism unit head Boniface Mwaniki said Adebolajo was believed to have been preparing to train and fight with the al Qaeda-linked Somali militant group.
Adebolajo was arrested with five others, and later deported, which is common in Kenya when involvement with terrorism is suspected.
According to SkyNews,Mr Mwaniki denied accusations that Adebolajo had been abused while in custody in Kenya.
The Foreign Office said in a statement: “We can confirm a British national was arrested in Kenya in 2010.” It provided consular assistance, as is normal for British nationals.
Adebolajo, 28, and Michael Oluwatobi Adebowale, 22, are suspected of murdering soldier Drummer Lee Rigby in Woolwich on Wednesday.
Three further men, aged 21, 24 and 28, who were arrested on Saturday, remain in custody, he added.
Counter-terrorism officers have also arrested a 22-year-old man in Highbury Grove, north London, on suspicion of conspiracy to murder.
Deputy Assistant Commissioner Stuart Osborne, head of the Counter Terrorism Command, said officers were working on the complicated investigation “tirelessly and painstakingly”.
“We are pursuing a significant amount of CCTV, social media, forensic and intelligence opportunities and have active lines of inquiry.”
The two suspects arrested at the scene of the killing remain in hospital and will be formally interviewed when it is possible to do so, Mr Osborne said. They were shot by police when they charged towards armed officers in Woolwich on Wednesday.
Mr Osborne continued: “This remains an ongoing investigation, focused upon public safety and identifying any others that may be involved.
“The investigation is progressing well, but there is still a lot more work to be done.”
He appealed for anyone with information should contact the confidential anti-terrorist hotline on 0800 789 321.
Home Secretary Theresa May has revealed that counter-terrorism officers from around the country have been brought in to boost the total number of police working on the investigation to 500.
The arrests came as Downing Street confirmed the launch of a new terror task force to crack down on extremism.
The group, comprising Cabinet ministers and top police and security service officials, will focus on radical preachers who seek out potential recruits in prisons, schools, colleges and mosques.
David Cameron, who is on a family holiday in Ibiza, has also said that the parliamentary Intelligence and Security Committee (ISC) will carry out an investigation after it emerged that Adebolajo and Adebowale – were known to MI5.
Authorities in France are also investigating whether the murder of Drummer Rigby was linked to an attack on a French soldier, who was stabbed in the neck in a busy shopping area in Paris on Saturday.
Meanwhile, the father of murdered schoolboy Damilola Taylor has revealed that he had acted as a mentor to Adebowale, who was known to his friends as Toby.
Richard Taylor told ITV News: “I was terribly shocked at what I saw that day. It’s a different Toby or Michael that I was seeing that day. I don’t believe it was anything Islamic.”
Mr Taylor, whose son was 10 when he was killed in London in 2000, said he had tried to help Adebowale after he was bullied at school and then became involved in drugs and gangs.
A 29-year-old man arrested on suspicion of conspiracy to murder has now been released on bail, police said.
Two women, aged 29 and 31, were released without charge after they were detained on Thursday on suspicion of conspiracy to murder.
A number of other people not directly involved with the attack have been charged over malicious comments made on social networking sites.
Drummer Rigby was hit by a car and then attacked with weapons including a knife and a meat cleaver.
The young father’s murder has provoked a backlash of anger across the country, with mosques being attacked, widespread racial abuse and comments on social media and a large increase in anti-Muslim incidents.
Up to 2,000 people took part in a EDL march in Newcastle on Saturday, which had been planned before Wednesday’s attack.
Police said there were no major incidents, with a small number of arrests related to alcohol and public order offences. Three people were held before the march for allegedly making racist tweets.