Mohegan — Lt. Gov. Nancy Wyman alluded to three letters of the alphabet that likely were on a lot of minds Monday during a "topping-off" ceremony that marked a milestone in construction of the Mohegan Tribe's $80 million Mohegan Sun Exposition Center.

But she wasn't about to utter them.

"They're three letters we don't want to talk about," Wyman said, addressing a hard-hatted gathering of officials and guests. "Let them stay out of our state."

Clearly, she was referring to M-G-M, as in MGM Resorts International, the Las Vegas-based behemoth that's threatening to hem in southeastern Connecticut's gaming tribes. It's building a nearly $1 billion resort casino in Springfield, Mass., and last week said it's eager to erect a more than $600 million casino on the Bridgeport waterfront.

MGM's also sought to sow doubt about the U.S. Department of the Interior's Sept. 15 response to amended state-tribal gaming agreements the Mohegans and the Mashantucket Pequots submitted for approval in August. The casino-owning tribes formed a partnership, MMCT Venture, to develop Connecticut's third casino, in East Windsor.

Now racing to launch that project, the tribes believe they have all the federal approval they need, Kevin Brown, the Mohegan chairman, said.

Interviewed following Monday's ceremony, Brown dismissed any suggestion that the Interior Department's Bureau of Indian Affairs had, in effect, failed to endorse the tribes' amendments.

Brown acknowledged that letters the tribes received from the Interior Department have led to some confusion. In them, an Interior official says department action "is premature and likely unnecessary."

U.S. Sen. Richard Blumenthal has termed the letters "ambiguous" and called for them to be clarified.

Brown, however, said the tribes are confident that under federal regulations, any response short of a definitive disapproval is considered to be an approval, or "deemed approved." Before that approval takes effect, Brown said, it must be published in the Federal Register, the daily journal of the U.S. government.

Federal regulations require that approvals of state-tribal gaming agreements — so-called compacts and amended compacts — be published within 90 days from the date the Interior Department receives them. In the letters, an Interior official indicates the department received the tribes' amended agreements on Aug. 2.

Brown said the 90-day period ends by Nov. 2. Attempts to obtain comment from BIA officials were unsuccessful.

In the meantime, MMCT is proceeding "aggressively" in East Windsor, where it has purchased the necessary land for the third Connecticut casino, and has chosen an architect and construction officials, according to MMCT officials. The casino is to be built on the site of an abandoned Showcase Cinemas building off Exit 45 of Interstate 91. The site is less than 20 miles from where MGM Springfield is scheduled to open next September.

MMCT is targeting an opening date for the East Windsor casino of December 2018.

The tribes' development agreement with the town of East Windsor calls for them to make a $3 million payment to the town no later than 15 months before the casino's opening.

That payment has not yet been made, "but it will be," Mitchell Etess, a member of MMCT's board and chief executive officer of Mohegan Gaming & Entertainment, said Monday. "It's been budgeted, funded and will be paid."

Brown questioned the viability of MGM's Bridgeport proposal.

"My initial response is: Where's the enabling legislation that says they're authorized to do it?" he said. "Until there's something that says it's legal, it can't happen."

If the state allowed MGM to open a Connecticut casino, it would free the Mohegans and the Mashantuckets, owners of Foxwoods Resort Casino, of their obligation to pay 25 percent of their casinos' slot-machine winnings to the state. Brown said the payments would stop as soon as the state and MGM reached an agreement — not when an MGM casino opened.

In between those two events, Brown said, the state could lose "between half-a-billion dollars and one-and-a-quarter billion dollars."

The Mohegan Sun Exposition Center, more than one-third complete, is on schedule for a June 2018 opening, in time for the 3rd Annual Barrett-Jackson Northeast Auction of collector cars. The center will encompass 240,000 square feet of space and include a 20,000-square-foot ballroom, 15 meeting rooms, 3,600 square feet of outdoor gathering space, a 1,260-square-foot executive boardroom with its own outdoor area and 20,000 square feet of additional entertainment tenant space.

Ray Pineault, Mohegan Sun's president and general manager, said the center will be the largest meeting space between New York and Boston.

b.hallenbeck@theday.com