A Frontier Airlines flight stayed on the taxiway at the Denver International Airport for nearly 50 minutes due to threatening notes that were found on board the plane.

Frontier Flight 3406 was on it’s way from the Seattle-Tacoma International Airport to DIA when crew members discovered two threatening notes on the aircraft, according to a statement from Frontier.

Law enforcement was immediately alerted and the flight landed safely at DIA.

A security sweep of the plane resulted in no findings of concern and the aircraft has been released back into service, Frontier said. Passengers were transported to the terminal by a bus to be reunited with their belongings or to continue with their travels, the airline said.

Denver Police Department officers assisted with deplaning passengers upon the aircraft’s arrival. The threat appears to be “unfounded at this time,” the police said in a statement.

The FBI is leading the investigation.

Denver Gazette news partner, 9NEWS, published this report.