Nissan chief Hiroto Saikawa
Saikawa said he could not reveal all details of what had happened because the case was still under investigation.
But the company accused Ghosn of having under-reported compensation amounts, misrepresented the company's investments and used company assets for personal use.
It also accused representative director Greg Kelly of having masterminded the fraud with Ghosn. The company dismissed Ghosn and Kelly on Thursday.
As someone who had devoted himself to the Nissan revival plan, "I am left with great resentment and dismay that is difficult to articulate", wrote Saikawa.
"I truly regret, and would like to apologise for, us betraying and completely letting down our customers, business partners and other stakeholders who supported us after our revival," he wrote.
The internal memo invited all staff to a "town hall" meeting on Monday with Saikawa to discuss the future of the company. - AFP