Executive A dug into the matter. Among his findings were issues with possibly unreported compensation.
According to one source, the investigations then diverged along two lines, one pursued by Executive A, the other by an individual in Nissan identified as "Secretary X." The source said he was not authorized to divulge the individuals' real names.
In the meantime, Executive A also was uncovering real estate dealings by Zi-A that raised questions. Nissan now claims that Zi-A was funneling funds into residences around the world for Ghosn's private use. According to Nissan, they included housing in Rio de Janeiro, as well as a home in Beirut, Lebanon.
The parallel investigations converged in June. Around that time, Executive A looped in another person identified as "Executive B." Together they approached a former prosecutor for legal advice on their findings.
The legal department suspected there might be a case of criminal breach of trust, and attorneys recommended that the matter be referred to prosecutors, one source said.
Saikawa was notified only after it was clear that prosecutors would pursue the case, the person said, adding that Saikawa stepped up Nissan's cooperation with prosecutors at that point.
Still unclear is the role of plea bargaining in all this. Nissan has said the suspected financial misconduct was uncovered by whistleblowers. But Japanese media have reported that the two Nissan informants used Japan's newly adopted plea bargain system.
Sources could not say whether Executive A or Secretary X took plea bargains. But both may have had some exposure to alleged wrongdoing, one source said.
Tokyo prosecutors last week indicted Ghosn not over the real estate issues, but for allegedly not reporting ¥4.87 billion ($43.3 million) in deferred compensation over the 2010-14 fiscal years. That was roughly half his actual executive compensation over those five years, they claim.
At the same time, prosecutors additionally arrested him on suspicion of also underreporting deferred pay by some ¥4.27 billion ($37.7 million) over the 2015-17 fiscal years.
Ghosn, who has been jailed in a Tokyo detention center since his Nov. 19 arrest, has not spoken publicly. But Japanese public broadcaster NHK says he denies any wrongdoing. If found guilty of the charges, Ghosn could face up to 10 years in prison.