The US carrier Southwest Airlines has grounded for inspection about 80 of its planes after a hole opened up in mid-air in the roof of one of its aircraft. Investigators have ruled out terrorism, but so far have not been able to establish the cause of the rupture. 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. One flight attendant was slightly injured during the incident on Friday. Witnesses said a couple of people aboard the Boeing 737-300 nearly passed out while scrambling for oxygen masks. There were 118 passengers and crew on board the 15-year-old plane. 'You can see sky' An FBI spokesman said terrorism was not suspected and "it appears to be a mechanical issue". Passenger Brenda Reese said the plane had just left Phoenix when she heard a "gunshot-like sound". "It's at the top of the plane, right up above where you store your luggage," she told the Associated Press by telephone from the plane. "The panel's not completely off. It's like ripped down... When you look up through the panel, you can see the sky." She said the plane had "dropped pretty quick". Another woman on board said: "They had just taken drink orders when I heard a huge sound and oxygen masks came down. "There was a hole in the fuselage about three feet long. You could see the insulation and the wiring. You could see a tear the length of one of the ceiling panels." Aviation officials said the pilot made controlled descent, dropping 8,000m (25000ft) from 11,000m in about five minutes. The National Transportation Safety Board said an "in-flight fuselage rupture" led to the sudden descent and drop in cabin pressure. "We do not know the cause of the decompression," Federal Aviation Administration spokesman Ian Gregor said. The same thing happened to another Southwest jet in 2009. Then, metal fatigue was the cause. in 1988, cracks caused a hole to open in an Aloha Airlines plane over Hawaii. In that incident, a flight attendant died. Source: BBC