Last September, two months before his arrest in Tokyo, then-Nissan Motor Co. Chairman Carlos Ghosn and the carmaker’s chief executive officer considered bringing in a new partner for the alliance with Renault SA and Mitsubishi Motors Corp., according to an email.

At the time, Ghosn was under pressure to make the three-way automobile alliance “irreversible.” In a message to Ghosn seen by Bloomberg, Nissan CEO Hiroto Saikawa wrote that he had been working over the summer “quietly by myself,” at his boss’s request, to find a structure that would be “acceptable for both sides.” He offered to discuss possibilities with Ghosn.

In the correspondence, Saikawa raised the possibility of bringing in another manufacturer as a fourth partner for the alliance. He didn’t identify any potential candidate. He wrote that expansion opportunities also included “acquisition of Chinese companies” for electric vehicles or connected services.

The email casts light on the private discussions between the two men on the way forward for the French-Japanese alliance, which together is one of the biggest automakers in the world. Internally, Saikawa had argued against a full merger and that Nissan should remain independent or be the dominant force in any deeper union. He told the Nikkei financial newspaper in April that Nissan wanted to maintain the three-way alliance.

Ghosn, who was chairman of all three companies, is now awaiting trial for financial crimes after spending 108 days in Tokyo Detention House. He has denied charges and blamed a “plot” against him by Nissan executives trying to prevent closer integration with Renault.

Nissan spokesman Nicholas Maxfield said the company doesn’t comment on, or confirm or deny, the content of internal communications. He said he wouldn’t comment on “matters potentially related to pending judicial processes.” Representatives for Ghosn’s family and Renault declined to comment.

In the message to Ghosn, Saikawa advises changing the alliance’s structure in 2019 “rather than wait.” Seven months before, Ghosn had pledged to cement the partnership, a promise that came after France, Renault’s most powerful shareholder, had demanded deeper ties with Nissan.

Yet a move for closer relations faced resistance from within Nissan, which feared Renault would gain even greater sway. Already their shareholding is lopsided, with Renault owning 43 percent of Nissan compared with the Japanese carmaker’s 15 percent stake in Renault. Their partnership has been under further strain since Ghosn’s Nov. 19 arrest.

In considering bringing in a fourth partner for the alliance, Saikawa’s email also highlights the pressure carmakers are under to grow amid an expensive shift by the industry to electric and self-driving cars.

Last week, the Financial Times reported that Renault wanted to pursue the idea of a merger with Nissan, before a tie-up with another partner, possibly Fiat Chrysler Automobiles NV.

Saikawa denied any deal-making talks, and said he was unaware of any such move by Renault. The French carmaker declined to comment and the government downplayed the report.

In a bid to restore trust, Renault and Nissan earlier this month unveiled a new board to govern their alliance.