Another Southwest Airlines Co. (LUV) plane in-air issue scared passengers after its cabin lost pressure.

The flight was on its way from Denver to Dallas on Saturday night when the cabin pressure began to drop, Southwest officials said, according to MarketWatch. Oxygen masks were released at about 9 p.m. as the plane was near Dallas.

Southwest Airlines said it did not know what caused the cabin pressure to fall but the jet was taken out of commission for a maintenance review.

Passengers were afraid during the incident because they did not know what was going on, according to local media reports.

“I had no idea what was going on or what the outcome was going to be,” passenger Glen Eichelberger told Dallas station KTVT. “I didn’t know if we were going to make it or not,” Eichelberger added. “There was no communication whatsoever from the flight attendants or from the cockpit as far whether we were in mortal danger.”

Southwest’s In-Air Scares

Last month, a Southwest jetliner suffered in-flight engine failure. The engine, which broke apart at more than 30,000 feet, sprayed metal prices through the fuselage and forced the jet to make an emergency landing in Philadelphia. One passenger died in the incident. (See also: Southwest Has First U.S. Airline Fatality in 9 Years.)

Then, earlier this month, a cracked window on a jetliner bound from Chicago to Newark, NJ, forced an airliner to make an unscheduled landing in Cleveland, Ohio. Southwest said that event was not an emergency as the plane did not lose pressure. That plane was also taken out of service for a maintenance review.

Southwest stock is down more than 5% in the past month.