The news conference was held at the MTS Centre, which in 2004 replaced Winnipeg Arena. The arena holds 15,000 fans but can be expanded, according to True North.

True North announced a drive to sell 13,000 season tickets before next season begins.The move was widely expected over the last three weeks, as news leaked out that the Atlanta Spirit Group, the owner of the Thrashers, could not find a buyer that would keep the Thrashers in Atlanta. The team was estimated to be losing more than $30 million a year.

“To our fans in Atlanta, we are not happy about leaving Atlanta,” Bettman said.

Mayor Kasim Reed of Atlanta said Tuesday that he was trying to block the move by finding local ownership for the Thrashers. It was the first public expression of such an effort by Reed, who has been criticized by fans for not fighting to keep the team in the city.

“We have put an extraordinary amount of effort in trying to put together a local ownership group, and we have really pushed hard,” Reed said. “It’s important to me, one, because I don’t like to lose, and, two, because I believe having the full accompaniment of major sports franchises says something about a city, and Atlanta is a world-class city.”

But the Atlanta Spirit Group issued a statement that appeared to make matters final.

“Earlier today, we, along with our partners, signed an asset purchase agreement to sell the Atlanta Thrashers to True North Sports and Entertainment,” the statement said. “It’s extremely disappointing to all of us that it became necessary after all other options were exhausted.”

The Thrashers went to Atlanta in 1999 as an expansion franchise. They made the playoffs only once, in 2007, and were swept by the Rangers. Their attendance consistently ranked among the bottom three in the N.H.L.