National safety inspectors have found evidence of “widespread cracking” and fatigue on the fuselage of a Southwest Airlines Boeing 737 that made an emergency landing in Arizona with a hole in the cabin, a government official said yesterday.

“Was the aircraft well maintained and should it have been maintained better? That is exactly why we are here, to look at why this problem occurred,” National Transportation Safety Board Member Robert Sumwalt said at a news conference broadcast from Yuma, Ariz., via Internet streaming.

As a result of the incident, Southwest has grounded part of its fleet for inspections.

The airline canceled 300 flights on Saturday and expected to cancel another 300 flights yesterday as the investigation continues into what caused the 5-foot-long, 1-foot-wide tear to develop during Southwest Flight 812 on Friday.

The flight from Phoenix to Sacramento landed at a military base in Yuma, Ariz., after the hole appeared suddenly at about mid-cabin.

The cancellations are likely to continue for the next few days, Southwest Airlines spokeswoman Whitney Eichinger said.

She said the airline would provide no additional comment due to the active investigation.