DALLAS — American Airlines canceled more than 900 flights Thursday to fix faulty wiring in hundreds of jets, marking the third straight day of mass groundings as company executives offered profuse apologies and travel vouchers to calm angry customers.

Other carriers operating similar aircraft also left passengers scrambling for alternatives as they too canceled flights to inspect the wire bundles at the heart of a renewed safety crackdown by the Federal Aviation Administration.

Alaska Airlines canceled 11 more flights early Thursday as it continued to inspect its nine MD-80 jets. Spokeswoman Caroline Boren in Seattle said Thursday's cancellations follow 28 Wednesday and three on Tuesday. The airline was working to accommodate all affected passengers, she said.

Midwest Airlines canceled at least 10 flights Thursday after it grounded all of its 13 MD-80 planes to inspect the same issue affecting American's aircraft. Midwest spokesman Mike Brophy said federal regulators cleared the planes to fly, but executives at the airline decided the aircraft should be re-inspected by Midwest personnel.

The problems could be just beginning. The latest checks are part of a second phase of audits being carried out by the FAA, which came under pressure from lawmakers after its inspectors were found to be too tax with Southwest Airlines Co. last year. That round of inspections runs through June 30.

American, the nation's largest carrier, has now scrubbed more than 2,400 flights since Tuesday, when federal regulators warned that nearly half its planes could violate a safety regulation designed to prevent fires.

That's more than one in three flights canceled over the last three days.

Daniel Garton, an executive vice president of American, said cancellations could extend into Friday.

A return to normal operations depends on how quickly mechanics can inspect and fix the wire bundles.

Airline spokesman Tim Wagner said Thursday that about 80 planes had been cleared to fly. Work had been completed on another 17 or 18 planes, but FAA inspectors had not yet signed off on those. The rest of American's 300 MD-80s were in various stages of inspections, Wagner said.

The fallout could be seen at airport ticket counters, where frustrated customers bickered with American employees, and on the stock market, where shares of American's parent company tumbled more than 11 percent Wednesday.

American estimates that more than 100 passengers would have been on each of those canceled flights. That means a quarter-million people have been inconvenienced this week.

Airline executives said they thought they had fixed the wiring two weeks ago, when they canceled more than 400 flights to inspect and in some cases fix the shielding around the wires in their MD-80 aircraft.

But this week, Federal Aviation Administration inspectors, who have been conducting stepped-up surveys of airline compliance with safety rules called airworthiness directives, said 15 of 19 American jets they examined flunked. That left the airline no choice but to ground all 300 of its MD-80s, the most common jet in American's 655-plane fleet.

"We have obviously failed to complete the airworthiness directive to the precise standards that the FAA requires, and I take full responsibility for that," Gerard Arpey, American's chairman and chief executive, said at an industry event in California.

Back at American's headquarters in Fort Worth, Garton apologized for the snafu and vowed the airline would fix the problem this time.

"We simply cannot put our customers through this again," he said.

Garton added that for American, "this certainly couldn't have come at a worse time." The airline faces record fuel prices and fear of a recession, and analysts forecast that its parent, AMR Corp., lost more than $300 million in the first three months of the year.

American declined to say how much it would spend on $500 travel vouchers and hotel rooms for stranded travelers and overtime for mechanics, or how much revenue it would lose by putting some displaced customers on other airlines. But Garton said it would be "significant."

Perhaps worried about that cost, investors on Wednesday sent AMR shares down $1.15 to $9.17.

American's problem — and Alaska Airlines' cancellation of 14 flights Wednesday to inspect its nine MD-80s — stems from an FAA order in 2006 covering the bundling of wires in the backup power system for the fuel pump of the MD-80. The FAA says improperly bundled wires could rub, leading to an electrical short or even fire.

American officials said the safety of their planes was never jeopardized, and the FAA said no serious incidents have been blamed on poorly bundled wires.

Some passengers took a jaundiced view toward American's promise to fix the problems.

Kathy Neer of Santa Fe, N.M., was caught up in both waves of cancellations to and from a vacation in Paris. She and her husband were stranded in Dallas on Tuesday on the final leg of their journey home. American gave the Neers a voucher for a hotel room and seats on another flight home Wednesday.

"They say our flight is leaving at 3:55 p.m., but do you think we trust them?" Neer said. "After being burned twice, we're a little skeptical."

AP Business Writer Adam Schreck in New York, and Associated Press writers Daisy Nguyen in Los Angeles and Jeff Carlton in Dallas contributed to this report.