NEW YORK—Southwest Airlines Co. canceled more than 60 flights Monday as safety personnel continued inspections for fuselage cracks in its fleet of Boeing Co. 737-300 jetliners, days after one of the planes sustained a ruptured hull while in flight.

Shares of Southwest fell 3% to $12.31 in early trading. For the year to date, the stock is down about 7.6%, versus a 5% climb in the benchmark Standard & Poor’s 500 Index.

Altogether the Dallas-based carrier is expected to cancel some 100 flights on Monday, according to a late Sunday news release, following the grounding of about 300 flights over the weekend. FlightAware, a travel service Web site, showed Southwest had canceled 64 flights as of early Monday.

Cracks have been found in at least three 737-300 jets since safety checks began, according to the National Transportation Safety Board, which is supervising the inspections.

Of 79 of the jets pulled for inspection, more than 30 have been returned to service, said Southwest spokesperson Whitney Eichinger. Inspections should be completed by Tuesday afternoon.

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