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DENVER - A flight bound for San Diego from Denver had to return to DIA on Monday morning after the crew declared an in-flight emergency.

Republic Flight 1557, operated as Frontier Airlines, suffered a "loss of pressurization" sixteen minutes after departing Denver, according to a spokesperson for Republic Airways Holdings.

The flight landed without incident, and there were no injuries reported.

FlightAware.com reports the flight descended from 24,400 feet to 18,600 feet over the course of a minute while flying over the Aspen area.

Passengers on the flight say they were notified by the flight crew that the plane had "15 minutes" in order to descend to a safer altitude.

9News aviation consultant Greg Feith says it appears there was no sudden and massive depressurization of the cabin because the oxygen masks did not drop during the flight. He also says the FAA will look into the matter because of the in-flight emergency.

Passenger Kathryn Kubala told 9News they were notified of the problem shortly after the plane, an ERJ-190, started to bank to the right over the mountains.

"It was surreal," Kubala said, while she waited for a 5 p.m. flight to San Diego. "We were flying extremely close to the mountains. We were all wondering why this was happening."

She said some passengers got out their cell phones in order to text loved ones while others started to quietly cry.

"There was no panic or anything," she said.

Bryan Swallow, a fellow passenger, joked that he got a good look at some Jeep trails that he'd like to explore in the future.

"It really was pretty scary," he said.

Ninety eight passengers and four crew members were on board the flight. Both Kubala and Swallow said they were extremely appreciative of the work of the crew members, some of whom were jostled about when the plane made its return to DIA at an unusually low altitude.