cnnee ryan zinke
Washington CNN  — 

Former Interior Secretary Ryan Zinke’s use of private email to conduct official business is now part of a Department of Justice criminal probe, a government watchdog said Tuesday.

The Interior Department inspector general wrote to members of Congress that it is now working with federal prosecutors.

“In consultation with DOJ and to avoid any interference with the criminal case, we have decided to combine the investigations,” read the letter from acting inspector general Gail Ennis.

It said the Justice Department probe began “last year,” around the time when two senior Democratic lawmakers, Reps. Raul Grijalva and Elijah Cummings, requested information from the inspector general on “many instances in which Secretary Zinke sent and received emails containing official business using his personal email account.”

In October, CNN reported that the Justice Department was investigating whether Zinke used his position for private gain.

Zinke resigned at the end of last year while several ethical reviews involving him were underway.