"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 had met specific Muslim militants, then asked Adebolajo 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 studio immediately after the interview was recorded.
"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."
It was not immediately possible to verify the information provided by Nusaybah, who said he had known Adebolajo for about a decade. MI5 does not publicly discuss its efforts to recruit informers.
In Kenya, Anti-Terrorism Police Unit chief Boniface Mwaniki said police were checking their records to confirm whether Adebolajo had been in their custody. Mwaniki rejected any suggestion that Adebolajo had been abused.
Nusaybah said Adebolajo converted to Islam around 2004. His account corroborates those provided by two Muslim hard-liners who said they also knew Adebolajo.
Anjem Choudary, a former leader of a banned British radical group called al-Muhajiroun, said Adebolajo was a Christian who converted to Islam around 2003. Choudary told the AP that Adebolajo participated in several of the group's London demonstrations before Britain outlawed al-Muhajiroun in 2010.
Omar Bakri Muhammad, another former al-Muhajiroun leader and radical Muslim preacher, said Adebolajo is a Nigerian who was born and raised in Britain. He said Adebolajo attended his London lectures in the early 2000s, but added he had not stayed in touch with the suspect since then.
Bakri fled London and resettled in Lebanon in 2005 after suicide attacks on London's public transit system killed 56 people, including four bombers.
"I don't know what Michael did since 2004 or 2005," Bakri told the AP. "Two years ago he stopped attending our open lectures and lessons as well as our activities."
The University of Greenwich confirmed Saturday that Adebolajo was a student there from 2003 to 2005 but dropped out.
Fewer details have emerged about Adebowale besides one reported brush with death as a teenager.
The Guardian newspaper, citing police and court records, reported Saturday that Adebowale was stabbed in 2008, when a man attacked him and two friends in a London apartment. One 18-year-old friend died and the attacker received a life sentence for murder, the newspaper said.
MI5 Director-General Andrew Parker is expected to deliver a preliminary report next week to Parliament's Intelligence and Security Committee detailing what the agency knew about both suspects and whether the domestic spy agency could have done anything to stop the attack.
The directors of Britain's foreign spy agency, MI6, and Britain's eavesdropping agency, GCHQ, also are expected to give reports on what intelligence they had on the two men.
Police earlier this week detained three others in connection with the murder probe. Two women were released without charge, and a 29-year-old man has been bailed pending further questioning.
Associated Press writers Paisley Dodds in London, Tom Odula in Nairobi, Bassem Mroue in Beirut and Shawn Pogatchnik in Dublin contributed to this report.