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An examination of a US plane that developed a hole in its cabin roof in mid-flight has found evidence of metal fatigue in the fuselage.

The hole caused a sudden drop in cabin pressure, and Flight 812 from Phoenix to Sacramento was forced to make a steep descent and emergency landing.

The Boeing 737-300, carrying more than 100 people landed safely in Arizona.

Owners Southwest Airlines cancelled 300 flights on Sunday to allow for inspections of 79 of its aircraft.

One flight attendant was slightly injured during the incident on Friday but no-one was seriously hurt.

There were 118 passengers and crew on board the 15-year-old plane.

Investigators said the rip began where two outer panels were riveted together, and that the area around it showed evidence of pre-existing cracking due to fatigue.

"We did find evidence of widespread cracking across this entire fracture surface," National Transportation Safety Board member Robert Sumwalt told reporters.

The plane is one of the oldest in Southwest's fleet and has made thousands of flights, but the company said it had undergone all required inspections.

Describing the incident, passenger Brenda Reese said the plane had just left Phoenix when she heard a "gunshot-like sound".

Witnesses said a couple of people nearly passed out while scrambling for oxygen masks.

Aviation officials said the pilot made a controlled descent, dropping 8,000m (25000ft) from 11,000m in about five minutes.

The same thing happened to another Southwest jet in 2009. Then, metal fatigue was the cause.

The company changed its maintenance programme as a result, but before that incident Southwest Airlines paid millions of dollars to settle charges it was skipping inspections.

In 1988, cracks caused a hole to open in an Aloha Airlines plane over Hawaii. In that incident, a flight attendant died.