PHOENIX — Flight attendants had just begun to take drink orders aboard Southwest Flight 812 on Friday when an explosion rocked the cabin.

Shawna Malvini Redden, a 28-year-old doctoral student, on Saturday recalled covering her ears then feeling a brisk wind rush by. Oxygen masks fell, the cabin lost pressure, and Redden, now suddenly lightheaded, fumbled to maneuver her mask in place as the pilot began a rapid descent from about 34,400 feet in the sky.

The 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 investigation.

Southwest, meanwhile, grounded 79 similar planes so they could be inspected; some 300 flights were canceled Saturday. Southwest operates about 170 of the 737-300s in its fleet of about 540 planes.