ST. PETERSBURG, Fla (Reuters) - 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 on Sunday.

"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 Sumwaltsaid at a press conference broadcast from Yuma, Arizona 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 said it expects to cancel another 300 flights on Sunday as the investigation continues into what caused the five-foot-(1.5-meter) long, one-foot- (35-cm) wide tear to develop during Southwest Flight 812 on Friday.

The flight from Phoenix to Sacramento landed at a military base in Yuma, Arizona, 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.

Sumwalt said he did not know of any additional problems with Southwest's Boeing 737 fleet.

Airline mechanics soon will saw out the portion of the plane skin that fractured during Flight 812, and it will be shipped overnight to Washington for further inspection, Sumwalt said.

It will take six to eight hours to remove the piece, which is expected to be about eight or nine feet long and two feet (0.7 meter) wide and weigh about three to four pounds (1.36 to 1.81 kg).

Sumwalt said there was evidence of what is known as "multi-site damage."

"We did find evidence of widespread cracking across this entire fracture surface," he said.

Sumwalt said the tear occurred in a concealed part of the plane and could not have been detected by the naked eye. He said the plane was most recently disassembled for heavy maintenance in March 2010, which would have been the last time such a fracture could have been detected.

Recorders from Flight 812 arrived at NTSB's headquarters on Saturday night, and they indicated that the decompression occurred approximately 18-1/2 minutes after takeoff, Sumwalt said.

The flight crew donned oxygen masks and declared an emergency.

The plane descended from its cruising altitude of 36,000 feet to 11,000 feet in approximately 4-1/2 minutes, Sumwalt said.

An inspection of the oxygen generators that supply oxygen to passengers indicated that the generators all had been activated, Sumwalt said.

A total of 931 Boeing 737-300s are operated by all airlines worldwide, with 288 of them in the United States, the Federal Aviation Administration said earlier.

(Reporting by Colleen Jenkins; Editing by Greg McCune and Eric Walsh)

Source: Reuters US Online Report Domestic News