Washington ? Northwest Airlines provided information on millions of passengers for a secret U.S. government air-security project soon after the Sept. 11, 2001, terrorist attacks, raising more concerns among some privacy advocates about the airlines’ use of confidential customer data.

The nation’s fourth-largest airline asserted in September that it “did not provide that type of information to anyone.” But Northwest acknowledged Friday that by that time, it had already turned over three months of reservation data to the National Aeronautics and Space Administration’s Ames Research Center. Northwest is the second carrier to have been identified as secretly passing travelers’ records to the government.

The airline industry has said publicly that it would not cooperate in developing a government passenger-screening program because of concerns that the project would infringe on customer privacy. But the participation of two airlines in separate programs demonstrates the industry’s clandestine role in government security initiatives.

In September, JetBlue Airways said that it turned over passenger records to a defense contractor and apologized to its customers for doing so.

Northwest said in a prepared statement Friday that it participated in the NASA program after the terrorist attacks to assist the government’s search for technology to improve aviation security. “Northwest Airlines had a duty and an obligation to cooperate with the federal government for national security reasons,” the airline said.

The carrier declined to say how many passengers’ records were shared with NASA from the period offered, October to December 2001. More than 10.9 million passengers traveled on Northwest flights during that time, according to the Transportation Department.

NASA documents show that NASA kept Northwest’s so-called passenger name records until September 2003. Such records typically include credit card numbers, addresses and telephone numbers.