Law enforcement and fire officials were investigating a potential threat on a plane at Denver International Airport on Monday afternoon, but police later said the threat was unfounded.Frontier Airlines said crewmembers discovered what they described as "threatening notes" on a plane.

Denver police Monday afternoon confirmed passengers on a flight arriving to Denver International Airport were evacuated due to a threat that "appears to be unfounded at this time."

Frontier Airlines announced an addition of 15 new flight routes this fall, including three at Dallas Fort Worth International Airport.