WASHINGTON - Prosecutors have begun presenting evidence to a grand jury in Washington in their probe of whether former interior Secretary Ryan Zinke lied to federal investigators, according to two individuals briefed on the matter.

The closed-door deliberations are focused on Zinke's decision not to grant a petition by two Indian tribes to operate a commercial casino in Connecticut, according to these individuals, who spoke on the condition of anonymity because grand jury proceedings are not public.

The Mashantucket Pequot and Mohegan tribes' push to run a gambling facility in East Windsor, Connecticut, had sparked a lobbying campaign by MGM Resorts International, a competitor that opposed the planned casino. The proposal was the subject of intense scrutiny at Interior and the White House during President Trump's first months in office.

The tribes allege that Zinke decided not to grant their application because of political pressure, and Interior's Office of Inspector General opened an investigation into the matter a year ago.

Investigators with the Interior Department's inspector general's office came to believe Zinke had lied to them in the course of that inquiry, and referred the matter to the Justice Department late last year.

Making false statements to federal officials constitutes a crime, but it can be difficult to prove because it requires prosecutors to show a person "knowingly and willfully" lied, rather than simply misstated a fact.

Both the Justice and Interior Departments declined to comment on the matter Friday.

Zinke did not respond to a request for comment. But in an interview with The Washington Post last month, he denied any wrongdoing in connection with the casino deal.

"The Department of Interior should not take a position on any activity outside the reservation that is not bound by law or treaty," he said.

"I sided with a principle that I didn't want to take a position on something that was off the reservation. I had multiple legal counselors' opinions about what was legal. The investigators may not have liked my answers, but they were truthful."

The Mashantucket Pequot and Mohegan tribes sought federal approval in 2017 to operate a commercial casino off their reservations, in an arrangement that would ensure the operation gave 25 percent of its slot revenue to the state of Connecticut.

MGM, which declined to comment this week, sought to block the petition on the grounds that it would provide the tribes with an unfair economic advantage. The casino giant has a gambling complex just 12 miles away from East Windsor, in Springfield, Mass., and has eyed opening a casino in Bridgeport.

Interior officials - including career staff and two Trump appointees - had tentatively signed off on the tribes' proposal in the summer of 2017. But in September the department declined to approve it, prompting a lawsuit by Mashantucket Pequot and the state of Connecticut. The tribe has questioned whether Zinke was improperly influenced by Rep. Mark Amodei, R-Nev., and then-Sen. Dean Heller, R-Nev., both of whom have received contributions from MGM Resorts International. The company ranked as Heller's second-biggest contributor between 2011 and 2016, according to the Center for Responsive Politics,giving him $57,450 during that period.

Attorneys representing the tribes and the state of Connecticut amended their complaint Wednesday in the U.S. District Court for the District of Columbia. The filing says that Heller called Zinke on Sept. 15, 2017, just hours before Interior issued its final decision, to pressure him not to approve the casino application. Heller could not be immediately reached for comment Friday

In September, U.S. District Judge Rudolph Contreras dismissed the tribe's initial claim. The Mohegan tribe withdrew from the litigation after Interior acknowledged the validity of its gaming agreement in June 2018.

Zinke, who voluntarily participated in two inspector general interviews about the matter, told The Post that he resigned at the start of the year because the investigations and "anger and hatred" from liberal opponents had created a poisonous atmosphere for him and the department he headed.

"If I had stayed, I would have become an angry person, and I didn't want that," he said.

As of Friday, Zinke had not been called to appear before the grand jury.

Witnesses before the grand jury have been asked whether anyone influenced Zinke's decision to rebuff the tribes' casino petition, according to the two individuals familiar with the proceedings. Prosecutors have also asked witnesses--who include Interior officials--about what sort of advice they provided Zinke in the course of his review of the application.

The Washington Post’s Matt Zapotosky and Josh Dawsey contributed to this report.