The focus of the Government Accountability Office investigation is not fraud, but waste, one of the four NASA officials told The Associated Press. The four - two still with NASA, two recently departed - asked not to be identified for fear of reprisals. Two said they had been questioned by the GAO, Congress' investigative arm.

A top GAO investigator, George Ogilvie, declined to comment.

O'Keefe said Thursday night that he was unaware of any such investigation, and that he had checked with NASA's inspector general, who also knew of no such probe.

He defended his use of government airplanes as a normal, necessary part of his job and said there were no abuses.

"To the extent there is such a thing under way, it will validate and confirm that the course of my entire career as a public servant and financial manager has been responsible, in this circumstance as well as every other previously," O'Keefe said.

A GAO probe was requested last June by Sen. Susan Collins, R-Maine, following testimony at spring hearings about "serious financial management problems" at NASA. Some of those problems predated O'Keefe's tenure.

NASA spokesman Glenn Mahone declined to comment, saying it would not be "proper or appropriate to comment on an ongoing investigation."

The officials familiar with the investigation told the AP that one area of interest to the GAO was O'Keefe's costly penchant for traveling on government airplanes, instead of flying commercially.

As a "basic principle," government employees are asked to use commercial flights, one of the officials said.

But O'Keefe "never, ever travels without going on a NASA airplane," another official said. And to justify the flights, O'Keefe often would fill the planes with other employees who might not have a need to travel.

The officials said another area the GAO is looking into is O'Keefe's "retreats" with subordinates far from NASA headquarters in Washington.

O'Keefe is leaving NASA after three years as the space agency's administrator and will become chancellor of Louisiana State University's main campus on Monday.

When they hired him late last year, university officials showered praise on him for his budget-conscious management skills. LSU system president William Jenkins said Thursday he was unaware of any GAO probe.

O'Keefe's new job at LSU will pay him $425,000 a year, nearly three times what he made at NASA.