Sun is suing Massachusetts gambling regulators over their decision to award the Boston-area casino license to a rival.
The Connecticut-based casino company on Wednesday joined a state Superior Court lawsuit filed in October 2014 by the city of Revere and a labor union representing workers at the Suffolk Downs horse racing track in Revere.
The lawsuit seeks to vacate the Massachusetts Gaming Commission's decision to award the license to Wynn Resorts, which plans a $1.6 billion casino on the Everett waterfront across from Boston.
Mohegan Sun had proposed a more than $1 billion resort at Suffolk Downs.
Mitchell Etess, CEO of the Mohegan Tribal Gaming Authority, said in an interview Wednesday that the company has never filed suit to challenge a casino license decision in any jurisdiction, but felt compelled to in this instance because of the many issues around what he called a "gravely flawed" decision.
"We were the most injured party," he said. "Yeah, this was bad for Revere and for those workers losing their jobs. But we were really not treated fairly at the end of the day and it was time for us to stand up for ourselves."
The suit also alleges that the Gaming Commission failed to impose at least a dozen conditions on Wynn that are mandated by law, including those relating to environmental remediation, neighboring community obligations, project labor and labor harmony agreements, pedestrian safety, investor suitability, audit requirements and a lottery agreement, among others.
Mohegan Sun joins a lawsuit that, among other things, alleges Wynn was given "preferential treatment" over Mohegan Sun.
The suit claims Wynn was allowed to resubmit significant portions of its application and provide substantially new information on the project's site design and traffic impacts, while no such opportunity was given to Mohegan Sun.
The Boston-area license is one of three gambling licenses state regulators have awarded so far.
MGM Resorts International is developing an $800 million resort casino in Springfield. Penn National is investing $225 million to expand the harness racing track in Plainville into a slot parlor.
A final resort casino license is designated for the southeastern portion of the state. The deadline for applications for that license is Jan. 30.