Flight 812 was forced into an emergency landing from 34,000 feet in the sky at a military base in Yuma, Ariz., due to the hole which measured about 3-feet long in the roof of the passenger cabin.
The pilots declared an emergency and brought the plane down 25,000 feet in less than five minutes.
Oxygen masks dropped as the cabin lost pressure.
I was "feeling my heart pumping out of my chest and watching people around me panic," said passenger Shawna Malvina Redden. "It was truly terrifying."
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 remained a mystery. Authorities from the National Transportation Safety Board arrived Saturday in Yuma to begin an investigation.
"Our mission is to determine not only what happened, but figure out why it happened so that we can prevent things like this from happening in the future," said Robert Sumwalt of the NTSB.
Southwest officials said the plane had undergone all inspections required by the FAA. They said the plane was given a routine inspection Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
As a result, Southwest flights at Southern California airports were canceled or delayed Saturday due to the incident.
"Customers may experience sporadic delays of up to two hours on some flights today," the company said in a statement. "Customers should check the status of their particular flight or rebook their trip on southwest.com before heading to the airport."
About half of the outgoing Southwest flights from LAX had been canceled or delayed. At Bob Hope Airport only a handful of flights were canceled.
Some passengers waiting to board were a bit apprehensive about getting on a plane after Friday's incident.
"It's pretty frightening when you think about it," said one passenger. "For the top of an airplane peeling away like a tin can."
But other passengers were just thankful that their flights weren't delayed. They said they won't let something like this keep them on the ground.
"I think in life you are going to have those situations that simply arise," said Ron Head from Vacaville. "And it is not the situation. It is how it is handled and how people react to it."
"I think flying has its dangers just like driving," said one passenger. "So just hope that everything goes okay."
The Associated Press contributed to this report.