DENVER — Federal officials are investigating after a Monday morning bomb threat evacuated a Frontier Airlines flight that landed at Denver International Airport, though no devices were found, police said.

Frontier crew members on Monday discovered two threatening notes onboard a flight headed from Seattle-Tacoma International Airport to Denver, airline spokesperson Rob Harris said in an emailed statement to The Denver Post.

It’s unknown what the notes said or how they were discovered.

When the plane landed, Denver police officers helped evacuate passengers and then searched the aircraft, according to the department.

“A security sweep of the aircraft resulted in no findings of concern and the aircraft has been released back into service,” Harris stated.

Denver police confirmed that officers cleared the plane, and that “the threat appears to be unfounded at this time.”

Passengers on the flight were taken to the terminal by bus to be reunited with their luggage or catch connecting flights, Harris said.

The FBI is investigating the incident, Denver police said. Federal officials did not respond Monday to requests for comment.

Two days earlier, an American Airlines flight was evacuated at DIA after the landing gear caught fire during takeoff.

American Airlines officials have not yet confirmed what caused the fire, but said the plane was taken out of service for inspection. One person was taken to the hospital with minor injuries, and five others were evaluated on scene.

Another American Airlines flight had to make an emergency landing at DIA in March, when one of its engines caught fire after taking off from Colorado Springs.

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