LONDON — Counterterrorism police on Saturday were questioning a friend of Michael Adebolajo, one of two suspects in the savage killing of a British soldier. The friend, Abu Nusaybah, was arrested immediately after he gave a television interview telling his story about how Adebolajo came to be radicalized.

In his interview, Nusaybah said Adebolajo became withdrawn after returning from a visit to study in Kenya, where he claimed he had been physically and sexually abused in detention. Nusaybah also alleged that the U.K.’s security services tried to recruit Adebolajo after he returned to Britain.

Adebolajo and another man are suspected of killing 25-year-old soldier Lee Rigby, hacking at his body with knives and a meat cleaver, on a London street in front of dozens of passers-by on Wednesday afternoon. Both suspects were shot by police at the scene and are hospitalized under guard in stable condition.

The BBC said Nusaybah was arrested by police outside its studios Friday night immediately after recording the interview.

“This interviewee had important background information that sheds light on this horrific event,” the broadcaster 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.”

Metropolitan Police confirmed that a 31-year-old man was arrested Friday night in London on suspicion of “the commission, preparation or instigation of acts of terrorism.” He is in custody at a London police station, police said, adding that the arrest was not directly related to the killing of Rigby.

Nusaybah told the BBC that he believed Adebolajo changed after he was allegedly detained and abused by security forces in a Kenyan prison cell last year. After that, Adebolajo became “less talkative ... he wasn’t his bubbly self,” Nusaybah said. He also said that Adebolajo told him that Britain’s security service, the MI5, followed him upon his return to the U.K. to find out if he knew certain individuals and then to ask if he would work for the security service.