The San Francisco Chronicle reported Monday Aoki, who committed suicide in 2009, passed along information to the FBI even as he gave members of the Black Power movement group some of their first guns and trained them how to use them.
Aoki later became a teacher, counselor and administrator in the Peralta Community College District.
Burney Threadgill Jr. told the Chronicle he was the FBI agent who recruited Aoki, a graduate of Berkeley High School, in the late 1950s to infiltrate activist groups.
"He was my informant. I developed him," Threadgill said. "He was one of the best sources we had."
Aoki's role inside the Black Panthers was discovered by Seth Rosenfeld while researching his book, "Subversives: The FBI's War on Student Radicals, and Reagan's Rise to Power," which will be published Tuesday by Farrar, Straus and Giroux.
Rosenfeld said Aoki had contended in a 2007 interview it wasn't true he was an informant, but added, "People change. It is complex. Layer upon layer."
Rosenfeld later obtained the FBI's records on Aoki through a federal Freedom of Information act request. Those records showed Aoki was an informant assigned the code number "T-2."
The Chronicle said an FBI spokesman declined to comment on Aoki, whose life in the Black Panthers was documented in a 2009 film, "Aoki" and a 2012 biography, "Samurai Among Panthers." Neither mentioned his work with the FBI.
The Panthers went on to have shootouts with police, including a 1967 incident involving co-founder Huey Newton in which one Oakland police officer was wounded and a second one killed, and a 1968 battle between Eldridge Cleaver and several other Panthers in which Cleaver and two Oakland police officers were wounded and Panther Bobby Hutton was killed.