Evidence emerged last night that one of the suspects involved in the killing of the British soldier Lee Rigby was well known to anti-terror police and the security services for at least three years before the brutal Woolwich attack. Michael Adebolajo was arrested in Kenya under suspicion of being at the centre of an al-Qa'ida-inspired plot in 2010, The Independent on Sunday can reveal.

He was one of seven men arrested by Kenyan police after landing on an island off the Kenyan coast in November 2010. Local press reports of the arrests referred to Mr Adebolajo as a "Nigerian with a British passport" who was "suspected of masterminding the racket". Police claimed the men were travelling to Somalia to join the ranks of the al-Shabaab terrorist group. His family claimed he was held in detention and tortured before being deported back to Britain without charge.

After the incident, members of his family said he was "pestered" by MI5 agents pressuring him to become an informant for them and infiltrate radical Islamic extremist groups. Relatives said other family members were also harassed and questioned by the UK authorities. In an exclusive interview with The IoS, Mr Adebolajo's brother-in law claimed constant demands to get him to spy on Muslim clerics might have pushed him over the edge.

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The allegation that MI5 knew of Mr Adebolajo's radical views for so long has increased the pressure on the intelligence services over their failure to recognise the scale of the threat he posed, amid the fall-out from the shocking killing of Drummer Rigby last Wednesday.

A friend of Mr Adebolajo has told the BBC that MI5 had attempted to recruit the suspected killer six months ago. It was also reported last night that Michael Adebowale, who was arrested alongside Mr Adebolajo following the soldier's killing, had been detained by police two months ago.

Sir Malcolm Rifkind, who leads the body that oversees the work of the intelligence services, said the organisations had "serious questions to answer". The MI5 chief, Andrew Parker, will provide a written report on the incident this week, before he is called before the Intelligence and Security Committee (ISC).

But Sir Malcolm, who is chairman of the ISC, pledged that the committee would also use new powers to force intelligence agencies to hand over all confidential documents relating to the case.

Abu Zuybyr, who is married to Mr Adebolajo's sister, Christiana, said last night that his brother-in-law had recently been "elated" following the birth of his child. But added: "Then things became a little strange."

Speaking from a café in the shadow of a mosque in an east Lancashire town, just before midday prayers, Mr Zuybyr said: "Why did he suddenly flip?" As family members struggled for explanations for Mr Adebolajo's actions, they speculated that pressure from the security services to turn informer may have pushed him to act.

"That is what the [Mr Adebolajo's] family is saying; that the secret service pushed him over the edge," his brother-in-law claimed.

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They insisted that Mr Adebolajo's character changed markedly in 2010, after a visit to Kenya – where, they say, he had gone to study Arabic and Islam with imams in mosques in Nairobi.

But an investigation by The IoS has revealed that Mr Adebolajo – officially described as "Mr Michael Olemindis Ndemolajo" – was one of seven youths arrested by Kenyan police on suspicion of trying to join the ranks of the al-Shabaab terrorist group in Somalia.

The young men had gone on a speedboat from Lamu island to Kizingitini, Pate island, where they were arrested by police who were waiting for them after a tip off. The group he was travelling with, which included two secondary-school boys, had been radicalised during weekly visits to a mosque in Mombasa, according to police sources.

They were thrown in jail and "the suspect from Nigeria" accused police of torturing him, according to local reports.

"We are being tortured by the police and we haven't eaten for two days now," he was quoted as saying. "We have been denied the right to talk to our family members and lawyer. We are being treated as criminals and we are innocent."

Kenyan media stated: "The Nigerian, Mr Michael Olemindis Ndemolajo, is said to have travelled from the UK to join the group."

He was kept in jail for several days before being deported back to Britain, "after it was established that his travelling documents were genuine and that he lacked a criminal record", according to The Nation newspaper. Mr Adebolajo's family said he had been told he would be hanged or beheaded, but after he had appeared in court he was freed to return to Britain. Relatives said they believed the release came after they had alerted local MPs and the Foreign Office. The decision has raised questions over official involvement in Mr Adebolajo's release – and the true extent of his connections with the intelligence services since he returned home to the UK.

Mr Zuybyr claimed his brother-in-law had been tortured violently, threatened with rape and his private parts had been grabbed.

Mr Zuybyr also alleged that, when Mr Adebolajo returned to Britain, he had been pestered by MI5 about informing for them. He added that other members of the family had been quizzed about Mr Adebolajo in an effort to put pressure on him.

Mr Adebolajo's elder brother, Jeremiah, had gone to Saudi Arabia to teach English, but had been arrested and harassed by the authorities about his brother. And Mr Zuybyr himself said that when he went to Yemen four years ago to learn Arabic with his new wife, he was rounded up and questioned at gunpoint.

He added: "When I came back to Britain, MI5 contacted me and showed significant interest in Michael. I was harassed for a while, with constant calls from people claiming to be from the FBI." Mr Zuybyr and his wife left their family in London and moved to east Lancashire, but he said MI5 had still tracked him down and questioned him about his brother-in-law.

Sir Malcolm made it clear last night that MI5 would be closely questioned about any suggestion that it had been in contact with the two men arrested in connection with Drummer Rigby's death.

He said: "I have been following all the news items and you can certainly assume that any of these serious allegations will be put to the intelligence agencies."

Colonel Richard Kemp, former commander of British forces in Afghanistan and a past chairman of the Government's Cobra intelligence committee, conceded that MI5 and the police could possibly have done more to prevent the shocking murder.

But, he added: "It would have required probably significant additional resources, so that they can increase the number of suspects they monitor. Even though these two were on their radar, they have to prioritise who they look at, because surveillance, monitoring and communications is all very expensive. And maybe they need more resources to do that if it is necessary to widen the net, which I think probably it is."

EDL marches in Newcastle

An estimated 2,000 English Defence League supporters paraded through Newcastle upon Tyne city centre yesterday as right-wing groups appeared to be gaining followers in the wake of Drummer Lee Rigby's death. Two weeks ago it was thought about 500 would attend.

Yesterday's demonstration, which was met by around 400 anti-fascist protestors, passed without major incident after 1,000 officers lined the streets, Northumbria Police said, adding that there were "a number" of arrests for drunkenness, or to prevent public order offences. The BNP (British National Party) has said it will march on Saturday in Woolwich, where the soldier lost his life last week.

The young father's killing provoked a backlash across the country, with many reported incidents of mosques being attacked and racial abuse.

Paul Cahalan