(CNN) -- Investigators found "widespread cracking" on the skin of a Southwest Airlines Boeing 737 that made an emergency landing in Arizona after a hole opened on top of the aircraft during flight Friday, a National Transportation Safety Board member said Sunday.

A flight attendant received minor injuries when the hole opened and the cabin depressurized 18 minutes after the plane took off from Phoenix bound for Sacramento, California, NTSB member Robert Sumwalt told reporters.

The flight data recorder indicated the plane was cruising at 36,000 feet when it depressurized, but it dropped to 11,000 within 4 1/2 minutes, Sumwalt said.

The pilot initially planned to return to Phoenix to land, but after the flight attendants reported seeing blue sky through the jet's roof he made an emergency landing at a military base in Yuma, Arizona, Sumwalt said.

The initial inspection found "clear evidence that the skin separated at the lower rivet line" where "the skin comes together on the aircraft," Sumwalt said.

The cracking would likely not be visible during routine inspections, but could have been seen when the plane underwent major maintenance and was disassembled in March 2010, Sumwalt said.

The hole that opened up was 5 feet long and 1 foot wide, he said.

Southwest mechanics cut away the section of the plane containing the hole and are sending it overnight to the NTSB laboratory in Washington, the board said.

Southwest grounded 79 of its planes for inspection for "aircraft skin fatigue" Saturday.

As of Sunday afternoon, 19 planes were returned to service after "intense inspection," the airline said in a statement. However, "small, subsurface cracks" or indications of cracks were detected in three other aircraft.

"Further evaluation and potential repairs will be necessary before those planes are returned to service," the statement said.

The additional cracks found Sunday were no more than 1/4 inch in size, the NTSB said.

The inspections of the remaining aircraft are expected to be completed by late Tuesday.

Boeing is sending a service bulletin to airlines with recommendations for how to inspect their planes for similar cracks, the NTSB said Sunday.

Southwest canceled about 300 flights Saturday and Sunday to accommodate the inspections. It advised customers to check the status of their particular flight or rebook their trip before heading to the airport.

Representatives from the NTSB, the Federal Aviation Administration, Boeing and the airline organized into teams to investigate the aircraft's structure, metallurgical aspects of the plane, flight data recordings and its maintenance records, Sumwalt said.

Meanwhile, some of the 118 passengers who were aboard the crippled Boeing 737 said they had feared for their lives.

"We were in shock," passenger Debbie Downey told CNN Saturday. "We were in row 16 and my husband and I could see blue sky ... the wiring, the cabling. It actually was terrifying."

She said, "a lot of people were crying and holding hands" but had trouble hearing due to the noise of air rushing through the plane's opening. "It was very, very scary."

"I heard a loud popping sound about three or four minutes before it blew open on us," passenger Greg Hansen said.

"(Then) a big explosion happened. A big noise, and from there, you felt some of the air being sucked out. It happened right behind me, in the row behind me and it covers about 2 1/2 rows," he said from seat 11C.

Hansen, 41, a regional sales manager for a biotech company, was flying home to Sacramento, California, from a business trip. Some people panicked and screamed as the blue sky and sun began to shine through the cabin in midflight, he said.

"Most people were just white knuckles holding onto the arm rests. The pilots did a great job and were under control to get us to a manageable level," he said.

But just behind him, Hansen said he could see the jagged edge of the aircraft where the rivets used to be.

"You can see the insulation and wiring. The interior ceiling panel was bouncing up and down with the air," he said.

"It was surreal, when you're riding in a modern aircraft. You're used to being enclosed and not having the window rolled down," he said.

Passenger Brenda Reese told CNN affiliates KCRA and KOVR she began to fear for her life.

"I was texting my sister to make certain that she told my kids that I loved them," Reese said.

Southwest Flight 812, which had a five-member crew, then made an emergency landing at Yuma Marine Corps Air Station/International Airport at 5:07 p.m. (7:07 p.m. ET.)

The Federal Aviation Administration said the captain made a rapid, controlled descent from 36,000 feet to about 11,000 feet after the cabin lost pressure.

Southwest said it provided a full refund, an apology and two complimentary round-trip passes on the airline for future flights.

The second flight landed in Sacramento on Friday night.

Boeing spokeswoman Julie O'Donnell declined to comment on possible causes of the incident.

"Boeing is committed to ensuring safe flight and to supporting our customers," the company said in a statement. "We are working closely with Southwest and are providing technical assistance to the U.S. National Transportation Safety Board as it investigates the incident."

A total of 288 Boeing 737-300s are currently operating in the U.S. fleet, and 931 operate worldwide, according to the FAA.

Journalist Rachel Elzufon contributed to this report.