(Jeff Haynes / Reuters)

The airline canceled about 600 flights over the weekend after a Boeing 737-300 jet sprung a 5-foot hole in the roof shortly after takeoff from Phoenix on Friday. The plane made an emergency landing. Southwest said no one was seriously injured.

Spokeswoman Brandy King says Southwest has inspected 33 similar planes and returned them to service and expects to complete checks on remaining grounded planes by late Tuesday.

King says two planes had cracks similar to those in the Phoenix jet and will be repaired before they fly again. A National Transportation Safety Board member tells the Associated Press a third plane was also found to have cracks developing.

A Southwest flight bound from Nashville to Baltimore-Washington International Marshall Airport lost a piece of its metal skin in 2009 and made an emergency landing in West Virginia.