Secretary of State Rex Tillerson used the alias "Wayne Tracker" to talk about climate change while serving as CEO of ExxonMobil, the fossil fuel giant that has come under fire over revelations that it sought to bury climate science for decades.

According to a letter sent to a New York state judge from New York Attorney General Eric Schneiderman's office, which is investigating the company's cover-up, Tillerson sent emails from that account to discuss climate-related issues from at least 2008 to 2015—and Exxon did not previously disclose the pseudonym. (Wayne is Tillerson's middle name.)

The letter asks the judge to order Exxon to explain whether documents from that account, and 34 other additional email addresses assigned to Exxon executives, had been saved.

It also notes that the company had produced dozens of documents from the Wayne Tracker email but did not clarify that it was used by Tillerson.

"If they had nothing to hide, then why the secret email account?"
—Jamie Henn, 350.org

"Exxon's top executives, and in particular, Mr. Tillerson, have made multiple representations that are at the center of [the Office of the Attorney General's] investigation of potentially false or misleading statements to investors and the public," the letter said.

In addition to climate change, the Wayne Tracker account was used to discuss "important matters" that the letter did not specify.

The secondary email account was discovered while Schneiderman's office was reviewing other Exxon documents. The team says the fossil fuel company has failed to hand over thousands of files that are relevant to the investigation.

Carrie Cohen, a former federal prosecutor who is not involved in the investigation, told Bloomberg on Monday that the development "raises a lot of questions" about whether Exxon complied with the subpoena to turn over its communications.

"It could be misleading to not tell the attorney general the actual owner of that email address," she said.

Jamie Henn, spokesperson for the climate group 350.org, added, "If they had nothing to hide, then why the secret email account?"