“It was the only source I know in the bureau where we had a source right in al Qaeda, directly involved,” Edward J. Curran, a former top official in the FBI’s Los Angeles office, told the court in support of a discrimination lawsuit filed against the bureau by his former agent Bassem Youssef.

Mr. Curran gave the testimony in 2010 to an essentially empty courtroom, and thus it escaped notice from the media or terrorism specialists. The Times was recently alerted to the existence of the testimony while working on a broader report about al Qaeda’s origins.