The nation's top transportation official ordered an investigation of two airlines that canceled hundreds of flights over the holiday weekend because of computer and staffing problems, stranding thousands of travelers.

U.S. Transportation Secretary Norman Mineta (search) asked the agency's inspector general to investigate the difficulties at US Airways (search) and Delta subsidiary Comair (search), which had to ground 1,100 flights on Saturday because of a computer problem.

Mineta said Monday in a letter to the agency's inspector general that people "must learn from the situations" to make sure they do not happen again.

US Airways Group Inc. was forced to cancel hundreds of flights and strand thousands of passengers between Thursday and Sunday because about triple the usual number of flight attendants called in sick.

Adding to US Airways' woes, an unusually high number of baggage handlers at Philadelphia's airport failed to report to work, the airline said, and the remaining staff became overwhelmed by a mountain of luggage.

Operations at the bankrupt company returned to near-normal on Monday, although workers were still trying to reunite people with their bags.

Kentucky-based Comair said it would be later this week before operations returned to normal. In a statement, Comair President Randy Rademacher promised cooperation with the DOT probe.

Union leaders at US Airways continued to blame thin staffing, denying that workers had staged a sickout to penalize the company for slashing their pay.

"We are so short staffed, if there is spit on the runway in Philadelphia, it causes a fiasco," said Teddy Xidas, president of the Pittsburgh chapter of the Association of Flight Attendants (search).

Of about 1,675 flight attendants scheduled to work Thursday, Friday and Saturday, about 300 called in sick daily, more than expected based on past holidays, US Airways spokesman Chris Chiames said. The carrier usually plans for about 100 such absences.

"To suggest it was somehow a staffing issue on our part is to shift attention away from the ... employees who chose to do this," Chiames said.

Alarm bells about a looming staffing crisis in Philadelphia apparently began sounding at least five days before Christmas and two days before snow began causing flight disruptions in Indiana and Ohio.

"I would not fly through [Philadelphia] this weekend," one Internet poster warned on Dec. 20 in a forum frequented by airline employees on the Web site USaviation.com.

The writer warned that a wave of US Airways baggage handlers were likely to skip work over the holidays, either because they were upset with the struggling company's latest proposals to trim pay and benefits, or because they were planning to retire and were using up spare sick days.

US Airways has been trying to cut costs to avoid liquidation.

Reservations and gate agents reluctantly approved a new contract Thursday that will cut their pay by 13 percent. Flight attendants are in the midst of phone balloting to decide whether to accept a contract with a 9 percent pay cut. Balloting is expected to end Jan. 5. The pilots' union previously ratified a new deal.

Two of the airline's employee groups are still negotiating.