Dallas ? American Airlines canceled more than 900 flights today 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.

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 late Wednesday afternoon that 60 planes had been cleared to fly, 119 were being worked on, and 121 planes had not yet been inspected.

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.