Phoenix ? Flight attendants had just begun to take drink orders when the explosion rocked the cabin.

Aboard Southwest Flight 812, Shawna Malvini Redden covered her ears, then felt a brisk wind rush by. Oxygen masks fell, the cabin lost pressure, and Redden, now suddenly lightheaded, fumbled to maneuver the mask in place.

Then she prayed. And, instinctively, reached out to the stranger seated next to her in Row 8 as the pilot of the damaged aircraft began a rapid descent from about 34,400 feet in the sky.

“I don’t know this dude, but I was like, ‘I’m going to just hold your hand,'” Redden, a 28-year-old doctoral student at Arizona State University, recalled Saturday, a day after her Phoenix-to-Sacramento flight was forced into an emergency landing at a military base in Yuma, Ariz., with a hole a few feet long in the roof of the passenger cabin.

No serious injuries were reported among the 118 people aboard, according to Southwest officials.

What caused part of the fuselage to rupture on the 15-year-old Boeing 737-300 was a mystery, and investigators from the National Transportation Safety Board arrived Saturday in Yuma to begin an inquiry.

NTSB board member Robert Sumwalt said investigators were going to cut a piece out of the fuselage, which would be studied for fracture patterns. He said they would also examine the plane’s black box and flight recorders, which arrived Saturday at NTSB headquarters in Washington, D.C.

Southwest, meanwhile, grounded about 80 similar planes so that they could be inspected, and said that as a result some 300 flights were being canceled Saturday. Airline spokeswoman Linda Rutherford said it was too soon to estimate the cost of grounding a portion of its fleet.

Southwest operates about 170 of the 737-300s in its fleet of about 540 planes, but it replaced the aluminum skin on many of the 300s in recent years, Rutherford said. The planes that were grounded Saturday have not had their skin replaced, she said.

“Obviously we’re dealing with a skin issue, and we believe that these 80 airplanes are covered by a set of (federal safety rules) that make them candidates to do this additional inspection that Boeing is devising for us,” Rutherford said.

Julie O’Donnell, an aviation safety spokeswoman for Seattle-based Boeing Commercial Airplanes, confirmed “a hole in the fuselage and a depressurization event” in the latest incident but declined to speculate on what caused it.

A total of 288 Boeing 737-300s currently operate in the U.S. fleet, and 931 operate worldwide, according to the Federal Aviation Administration. “The FAA is working closely with the NTSB, Southwest Airlines and Boeing to determine what actions may be necessary,” the FAA said Saturday.