Federal aviation officials are investigating the cause of a 12-inch-long crack that developed in the top of a Southwest Airlines Co. jet during flight, leading to a gradual loss of cabin pressure.

Pilots on Southwest Flight 1685 from Las Vegas to Boise, Idaho, descended to 22,000 feet Monday after receiving a pressure-drop alert for the cabin, the Dallas-based carrier said Friday. The action allowed the plane to maintain a safe pressure, the Federal Aviation Administration and Southwest said. Oxygen masks, which automatically pop out of ceiling panels if needed, didn’t deploy.

It was the third time such cracks have developed on the airline’s 737s, Southwest said. The two earlier ruptures were found during inspections conducted every 1,500 flight cycles and mandated by the FAA to detect cracks and missing or loose fasteners. It’s not clear yet whether additional inspections on 737s will be ordered.

All of Southwest’s aircraft are compliant with required inspections, the carrier said.

The Boeing Co. 737-700 involved in the Tuesday flight continued to its destination and landed without incident, Southwest said. None of the 123 passengers on board were injured. The plane is an older version of the 737, not the newer Max model that has been grounded for a year.

An older 737-300 operated by Southwest suffered a similar, but more extreme, incident on April 1, 2011, when a five-foot section of the roof tore loose at 34,000 feet. The cause was a manufacturing defect in how the skin was attached to the plane, the National Transportation Safety Board concluded. Southwest since has retired all its -300s.

The rupture and FAA investigation were reported earlier by the Wall Street Journal.