Counterterrorism police have questioned a friend of Michael Adebolajo, one of two suspects in the killing of an unarmed British soldier, a savage attack that has horrified Britain.
The friend, Abu Nusaybah, was arrested on Friday evening, immediately after he gave a BBC Television interview describing how Adebolajo may have become radicalised and alleging that Britain's security services tried to recruit him.
Police said Nusaybah was wanted on suspicion of involvement in unspecified acts of terrorism.
Nusaybah said Adebolajo became withdrawn after he visited Kenya, where he claimed he had been arrested and then abused both physically and sexually while in jail.
Nusaybah claimed that Britain's domestic spy agency, MI5, approached Adebolajo to recruit him upon his return to Britain about six months ago.
Adebolajo, 28, and Michael Adebowale, 22, are suspected of killing soldier Lee Rigby by hacking his body with knives and a meat cleaver in front of dozens of passersby on Wednesday in the southeast London district of Woolwich.
The horrific scene was recorded on witnesses' cellphones, and a video has emerged showing one of the two suspects making political statements and warning of more violence as the soldier lay on the ground.
Police shot both men as they arrived minutes after Rigby's slaying. Both suspects remain under armed guard at two London hospitals.
Questions abound over what could have led the two men to attack Rigby, a 25-year-old ceremonial military drummer and machine-gunner who had served in Afghanistan and was off duty when he was walking near his barracks.
Nusaybah's interview offered one possible narrative. He said Adebolajo's behaviour changed after he allegedly suffered abuse at the hands of Kenyan security forces.
"Although that change wasn't necessarily one that became overt, aggressive or anything like that, he became ... less talkative. He wasn't his bubbly self," Nusaybah told the BBC.
He said MI5 agents approached Adebolajo after he returned to Britain and initially asked him if he was willing to act as an informer.
"He was explicit in that he refused to work for them," Nusaybah said.
The BBC said police arrested Nusaybah outside its studios Friday night immediately after recording the interview.
"This interviewee had important background information that sheds light on this horrific event," the BBC said in a statement. "And when we asked him to appear and interviewed him, we were not aware he was wanted for questioning by the police."
London police confirmed that a 31-year-old man was arrested on Friday night on suspicion of "the commission, preparation or instigation of acts of terrorism".
Police declined to identify Nusaybah by name or provide further detail.
The far-right British National Party is holding a rally in Woolwich on Saturday June 1, to call for the government to deport what it calls "Muslim hate preachers".
"A line has been drawn in the sand and it signals the beginning of the civil war we have predicted for years," the BNP said in a statement.