US Secretary of State Rex Tillerson, the former head of Exxon Mobil, used an alias email address while at the oil company to discuss information related to climate change, the New York attorney general says.
Eric Schneiderman says Mr Tillerson used an account named "Wayne Tracker" for at least seven years.
Wayne is Mr Tillerson's middle name.
Mr Schneiderman is investigating whether Exxon misled investors and the public about climate change.
In a letter to a New York state judge overseeing the investigation, the attorney general said the company had not previously disclosed the account, which was also used to discuss other issues.
The attorney general also said Exxon had produced some 60 documents bearing the "Wayne Tracker" account but never said it was used by Mr Tillerson.
Mr Schneiderman said Exxon had failed to give his office all documents covered by a court order in response to the investigation.
The state department declined to comment.
Alan Jeffers, a spokesman for Exxon Mobil, said the account was created for "secure and expedited communications" between Mr Tillerson and senior executives over a range of "business-related topics".
He added: "The very fact the attorney general's office has these emails is because they were produced in response to the subpoena."
Exxon has been accused of trying to cover up the risks of climate change and lying to the public.
Mr Tillerson resigned as chairman and chief executive of the company last year after his appointment as secretary of state in President Donald Trump's administration.