MI5 tried to recruit Woolwich attack suspect Michael Adebolajo, it has been claimed.
Abu Nusaybah, who claimed to be a childhood friend of the 28-year-old suspect, told the BBC's Newsnight that Adebolajo had been approached by MI5 six months ago and asked if he would work for the security service.
Nusaybah told the BBC Adebolajo had rejected the approach, but said he knew certain individuals MI5 was interested in.
He was arrested at the BBC's headquarters following the interview.
The Metropolitan Police said a 31-year-old man was arrested in relation to terrorism offences and search warrants were executed at two addresses in east London.
The arrest was not directly linked to Drummer Lee Rigby's death, the Met said.
The BBC said it was impossible to verify Nusaybah's claims.
He told Newsnight Adebolajo was being "harassed" by the security services.
He said: "His wording was, 'They are bugging me - they won't leave me alone.'
"He mentioned initially they wanted to ask him if he knew certain individuals.
"But after him saying that he didn't know these individuals, what he said was they asked him if he would be interested in working for them.
"He was explicit in that he refused to work for them but he did confirm he didn't know the individuals."