ATA Airlines Inc., a U.S. Midwest regional carrier that said last month it had to cut flights to survive, filed for bankruptcy for the second time in four years and said it will shut down all operations.

The airline sought Chapter 11 protection from creditors yesterday in U.S. Bankruptcy Court in Indianapolis, where ATA is based. The unexpected cancellation of a contract for military charter flights made it impossible get financing to stay in business, the company said in a statement today.

ATA said it will no longer honor tickets or reservations and customers should seek alternative arrangements. The company has 2,230 employees, about a fourth of them in Indianapolis, court papers show.

"We deeply regret the disruption and hardship caused by the sudden shutdown of ATA, an outcome we and our employees had worked very hard and made many sacrifices to avoid," said Doug Yakola, chief operating officer of ATA, in the statement.

The closely held company listed debts of $100 million to $500 million and assets of $100 million to $500 million in court papers.

The airline said in a March 6 statement that it planned to quit operating from its hub at Midway Airport in Chicago and shut down routes to Cancun and Guadalajara, Mexico, to save money.

ATA came out of bankruptcy in February 2006, after firing workers, cutting flights and ending aircraft leases. It was acquired by private equity fund MatlinPatterson Global Opportunities Partners LP.

Southwest Airlines Co., the largest low-fare carrier, had invested $20 million in ATA to help it operate during bankruptcy. Southwest received ATA gates at Chicago's Midway Airport and entered into a marketing alliance with ATA as part of its investment.

During its first bankruptcy, filed in 2004, the carrier ended service in Denver, San Juan, Puerto Rico, and Indianapolis.

The case is In re ATA Airlines Inc., 08-3675, U.S. Bankruptcy Court, Southern District of Indiana (Indianapolis.)