While he was chief executive of ExxonMobil, current secretary of state Rex Tillerson used an alias email account for eight years to discuss climate change and the risks it posed to the company's business, according to investigators for New York Attorney General Eric Schneiderman.

Those investigators say the company concealed the shadow emails despite a 2015 subpoena for Tillerson's communications issued as part of a sweeping investigation of the oil giant in connection with possible financial fraud.

Schneiderman's office disclosed the existence of the email account assigned to Tillerson on Monday in a letter to Judge Barry Ostrager, which accused Exxon of failing to turn over all relevant documents required by the subpoena.

Tillerson, whose middle name is Wayne, used an email address on the Exxon system under the pseudonym "Wayne Tracker" from at least 2008 through 2015, investigators say. The company has turned over a handful of the emails, but New York authorities believe a much larger trove exists.

"Mr. Tillerson used this secondary email address to send and receive materials regarding important matters, including those concerning to the risk-management issues related to climate change that are the focus of OAG's [office of the attorney general] investigation," according to the letter.

"[N]either Exxon nor its counsel have ever disclosed that this separate email account was a vehicle for Mr. Tillerson's relevant communications at Exxon, and no documents appear to have been collected from this email account."

The accusation comes amid heated and drawn-out court battles between Exxon and Schneiderman and Massachusetts Attorney General Maura Healey, who are both investigating whether the company misled investors for years about the possible impact of climate change on its business.

Investigators became suspicious in the last several days that Exxon wasn't telling them everything when they came across a reference to the Wayne Tracker email account buried in 60 of the 415,000 documents Exxon has turned over.

Investigators also say 34 other email accounts assigned to top Exxon executives, board members, or their assistants also exist but have not been turned over, according to the letter.

The Tillerson emails as well as those from the other Exxon executives are relevant, Schneiderman's investigators say, because they have made multiple representations of potentially false or misleading statements to investors and the public.

"As the investigation has progressed, documents Exxon has produced from other custodians have confirmed the close involvement of these key management individuals in the company's development and implementation (or lack thereof) of its claimed risk-management policies," according to the letter signed by John Oleske, senior enforcement counsel for Schneiderman.

"This has only increased OAG's need for complete and transparent disclosure of relevant documents reflecting the role of these top managers, as OAG determines which witnesses to seek testimony from as the investigation proceeds to its next logical phase."