Meanwhile, three more Southwest Airlines jetliners have small, subsurface cracks that are similar to the cracks suspected of playing a role in the fuselage tear of a Boeing 737-300, causing the aircraft to lose pressure and forcing a frightening emergency landing, officials say.

The 1 1/2 meter-long hole tore open in the passenger cabin roof area shortly after the plane left Phoenix for Sacramento, California, Friday afternoon. None of the 118 people aboard was seriously hurt as the plane descended from 10,500 meters to a military base in Yuma, 240 km southwest of Phoenix.

Since then Southwest grounded its 79 other Boeing 737-300s and began inspecting them.

Sunday night, another Southwest Boeing jet was diverted, this time because of a burning electrical smell in the passenger cabin, Southwest and Los Angeles International Airport officials said.

The plane carrying 142 people was en route from Oakland, California, to San Diego, when it made an unscheduled landing in Los Angeles, where passengers changed planes and continued on their journey, Southwest spokeswoman Whitney Eichinger said. No one was hurt.

She said the cause of electrical smell is being investigated, but it “was completely unrelated to the issue in Arizona.” She said Sunday's aircraft was also a Boeing 737 but she didn't know if it was the 737-300.

In its statement on the inspections, Southwest said Sunday that two planes have been found to cracks similar to those in the stricken aircraft and will be evaluated and repaired before they are returned to service. A National Transportation Safety Board member told The Associated Press later that a third plane had been found with cracks developing.

The other 19 aircraft inspected so far showed no problems and will be returned to service.

Checks on the remaining jets are expected to be completed by late Tuesday, the airline said. That means flight cancellations will likely continue until the planes are back in the air. About 600 flights in all were canceled over the weekend after Southwest grounded 79 of its planes.

The cracks found in the three planes developed in two lines of riveted joints that run the length of the aircraft. The agency is focusing its probe on the area of the cracks but has not determined that the cracks caused the rupture.

The Federal Aviation Administration declined to say if it was requiring other operators to check their aircraft for similar flaws.