Connecticut joined the Mashantucket Pequot and Mohegan tribes in suing the U.S. Department of the Interior late Wednesday afternoon, a move aimed at jump-starting the dormant East Windsor casino project.

In the suit, filed in U.S. District Court for the District of Columbia, the state and the tribes seek to compel the Interior Department to act on gaming-agreement amendments Gov. Dannel P. Malloy signed with each of the tribes last summer. The department and Ryan Zinke, secretary of the Interior, are named as defendants.

Malloy’s office announced the filing, which had seemed to grow increasingly inevitable in recent weeks.

The suit contends that because the department failed to act on the amendments within 45 days of their submission, the amendments are “deemed approved,” under the Indian Gaming Regulatory Act. The suit says the court should require the Interior Department to adhere to federal regulations and publish notice of its approval in the Federal Register, the daily journal of the U.S. government.

“State law requires that these compact amendments are in fact approved,” Malloy said in a statement. “That’s why I have asked the attorney general to file this action. We need clarity and certainty with respect to this issue.” He said publication of the Interior Department’s approval “is necessary in order for the amendments to be legally effective and enforceable.”

The Interior Department's inaction has threatened to delay development of the East Windsor casino, a $300 million project to be built on the site of a former Showcase Cinemas building off Exit 45 of Interstate 91. The project is supposed to protect the tribes’ respective casinos — Foxwoods and Mohegan Sun — from the competitive impact of MGM Springfield, a nearly $1 billion resort casino under construction in western Massachusetts.

MGM Resorts International, which has vowed to pursue litigation to stop the East Windsor project, issued a response to the lawsuit’s filing Wednesday night.

“The Department of the Interior has clear authority to return submissions exactly as it did in its September 15 ruling, which ‘maintain[s] the status quo,’” Uri Clinton, the company’s senior vice president and legal counsel, said. “The Tribes are wrong when they insist that their amendment is deemed approved by default. Approving a new, third casino is the opposite of maintaining the status quo. Precedent shows that an amendment is not ‘deemed approved’ when Interior returns it, plain and simple. And no lawsuit, not even one backed by the Governor, changes those basic facts.”

State and federal lawmakers promptly issued statements in support of the suit.

“This legal action is absolutely necessary because of the irresponsible and illegal failure of federal officials to do their job and approve the tribal plan,” U.S. Sen. Richard Blumenthal, D-Conn., said in a phone interview. “They have a duty to Native American tribes — in fact, it’s a trust responsibility — to take action and approve measures that are in the interest of federally recognized tribes.”

Blumenthal said his view is that “the law is crystal clear” and that the suit presents “a clear-cut issue” for the court.

“The Department of the Interior must follow the law,” he said. “There’s an economic interest here, not only for the tribes but for the state economy. There’s a profound public interest in the court acting promptly and prudently.”

In Sept. 15 letters to Malloy and the chairmen of the tribes, an Interior Department official had indicated that department action on the gaming amendments was “premature and likely unnecessary.” Curiously, two Nevada lawmakers — U.S. Sen. Dean Heller and U.S. Rep. Mark Amodei, both Republicans — were copied on the letters. MGM Resorts, based in Las Vegas, is known to have contributed to the Heller and Amodei campaigns.

“I have no inside information (on the connection between MGM Resorts and the Nevada lawmakers) but the facts that have been made public show a responsiveness to Nevada, which is MGM’s base,” Blumenthal said. “All I can do is apply common sense, and say maybe the Department of the Interior was acting more based on politics than on the law.”

Blumenthal also joined U.S. Sen. Chris Murphy, D-Conn., and U.S. Reps. John Larson, D-1st District, and Joe Courtney, D-2nd District, in a statement backing the suit.

“The law is clear — the Department of the Interior has approved the amendments and must now publish such approval,” the lawmakers said. “Court action to require publication of approval is necessary because Interior’s inaction violated federal law. Interior must follow the law and do what is best for both the tribes and Connecticut.”

State Sen. Cathy Osten, D-Sprague, whose district includes Foxwoods and Mohegan Sun, called the lawsuit “good news for eastern Connecticut,” with “more than 10,000 jobs in the region hanging in the balance.”

“We passed a bipartisan bill nearly six months ago to defend ourselves against an outside effort to destroy jobs and decimate one of the most important industries in our state,” she said. “The federal government is now long past its deadline to act on the agreements that the representatives of the people of Connecticut have deemed vital to the health and prosperity of our state.”

In statements, both tribal chairmen indicated the Interior Department’s failure to act on the gaming amendments forced the state and the tribes to sue.

“The Department of the Interior has a responsibility to Native American tribes, and their failure to act on this issue sets a very dangerous precedent for Indian Country across the United States,” Kevin Brown, the Mohegan chairman, said. “By not doing so, the department is in clear violation of federal law.”

“We hope for a quick resolution as we move forward with our plans to build a new facility in East Windsor,” said Rodney Butler, the Mashantucket chairman.

A spokeswoman for the attorney general’s office said the office could not speculate on how long it might take the court to act on the suit.

b.hallenbeck@theday.com