Abu Nusaybah was arrested under terrorism legislation after making allegations that MI5 had wanted Adebolajo to spy for them, in a television interview for BBC Newsnight yesterday.
Nusaybah was dramatically arrested in the BBC office after giving the interview.
According to the Guardian, Adebolajo is believed to have gone to a lawyer to complain of harassment by Britain's domestic intelligence service MI5, who are already under pressure for knowing about the 28-year-old but not assessing him to be a major terrorist threat.
In the TV interview, Nusaybah also attributed Adebolajo's radicalisation to his alleged ill-treatment in Kenya, a few years ago.
Nusaybah said he had known Adebolajo since they grew up together at Romford in Essex.
His arrest takes the total number to six but Nusaybah's is the first under counter-terrorism laws with the rest detained under regular criminal laws.
Nusaybah was arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
Police have released two women they had arrested as part of the investigation on Thursday, although a 29-year-old man remained in custody on suspicion of conspiracy of murder.
Meanwhile, it also emerged that the mother of the second suspect, 22-year-old Michael Adebowale, had tried desperately to turn him away from extremism.
Juliet Obasuyi, a 43-year-old probation officer, went to a friend for help about nine months ago after her son dropped out of university.
Adebowale and Adebolajo are thought to have met at Greenwich University when they converted to a radical form of Islam.
Adebowale's father, originally from Nigeria, now works for the Nigerian High Commission in a flat in north London, which was also raided as part of investigations on Thursday morning.
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