Congress' investigative arm is looking into Sean O'Keefe's (search) tenure as NASA chief, including whether he misused government airplanes and went on too many expensive getaways with underlings, former and current senior NASA officials say.

The focus of the Government Accountability Office (search) 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.

A top GAO investigator, George Ogilvie (search), declined to comment.

O'Keefe said Thursday night that he was unaware of any such investigation, and that he has 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.

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. Late Thursday, he held a news conference on campus to discuss the reported GAO probe.

When they hired him late last year, university officials showered praise on him for his budget-conscious management skills.

At Thursday's news conference, LSU system president William Jenkins echoed O'Keefe's comments, saying he too was unaware of any GAO investigation of O'Keefe.

Earlier in the day, 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," one of a half-dozen small jets the space agency shares with other agencies, another official said. And to justify the flights, O'Keefe often would "fill the planes with ballast," the official said -- other employees who might not have a need to travel.

"A lot of the times, at the last minute, Sean would be looking for people to put on the plane. We would call it baggage," an official said.

The officials said another area the GAO is looking into is O'Keefe's "retreats" with subordinates far from NASA headquarters in Washington, in contrast to the more sparing practices of his predecessor.

One official said these sometimes took place in Monterey, Calif., and at Syracuse University in upstate New York, where O'Keefe taught.

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

C. Stewart Slack, chairman of LSU's board of supervisors, said he knew nothing about the investigation. "Anytime you've got somebody in a position like that, I'm sure there's somebody who wants to take a shot at him," Slack said Wednesday.

The GAO began an investigation into NASA last June at the request of Sen. Susan Collins, R-Maine, following testimony at spring hearings about financial problems at the agency. Some of those problems predated O'Keefe's tenure.