The Department of Justice is investigating whether former Interior Secretary Ryan Zinke violated federal law by using private email to conduct official government business, according to a letter made public Tuesday by two high-ranking House Democrats.

The Interior Department’s Office of Inspector General launched the email probe last year and has since folded it into a larger criminal investigation it’s conducting with the DOJ, said Gail Ennis, the acting inspector general of the Interior Department, in a letter to House Natural Resources Committee Chairman Raúl Grijalva (D-Ariz.) and Oversight Committee Chairman Elijah Cummings (D-Md.).

Grijalva and Cummings requested the Office of Inspector General investigate the matter in April 2018.

Zinke resigned as interior secretary in January under a cloud of ethics scandals. He faced at least 18 formal investigations, several of which are ongoing. The larger DOJ probe centers on Zinke’s ties to a real estate deal in his hometown of Whitefish, Montana, The New York Times reported late last year. The development project involves a foundation Zinke started and was president of before taking a job in the Trump administration. The former chairman of energy giant Halliburton financially backed the project, Politico first reported.

Zinke’s legal troubles don’t end there. As of February, a grand jury was reviewing evidence from prosecutors to determine if the ousted agency chief lied to federal investigators about a Native American casino project in Connecticut, The Washington Post reported. A month after news of the grand jury probe broke, the Interior Department approved the casino deal ― an abrupt departure from its previous decision to block it.

The DOJ investigation is “part of restoring public trust in government, which has suffered from the intentional destruction of the Trump years,” Grijalva said in a statement Tuesday.

“Leaving office half a step ahead of the law doesn’t wipe the slate clean,” he said. “It’s imperative that political appointees at the Department of Justice allow this investigation to continue unimpeded regardless of the risk to Mr. Zinke or other Trump officials.”

In an interview with Bloomberg last week, Zinke dismissed the many investigations he faced as “b.s.” and falsely claimed he’d been completely cleared in all of them.

“There was no substance; there never was,” he told Bloomberg.

The Office of Inspector General concluded last month that Zinke violated federal law when he tweeted a photo of himself wearing socks that feature President Donald Trump’s face and the words “Make America Great Again.” And last year, the agency watchdog determined that Zinke violated government travel policies by bringing his wife on taxpayer-funded trips and by asking staff to explore making her a department volunteer, a move that would have legitimized her travel.