Frontier Airlines plane at Denver International Airport evacuated; police say "threatening notes" were "unfounded"

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. Passengers reported seeing police officers and bomb units, including dogs, on and around the plane after it landed.

Frontier flight 3406 from Seattle was evacuated after landing at Denver's airport around 11:15 a.m., about an hour after it was originally scheduled to land, according to Frontier's flight tracker.

"On Monday, July 28, as Frontier flight 3406 was en route from Seattle-Tacoma International Airport (SEA) to Denver International Airport (DEN), crewmembers discovered two threatening notes on the aircraft," a Frontier spokesperson said in a statement. "Law enforcement was promptly alerted, and the flight landed safely at DEN. Passengers are being transported to the terminal by bus while law enforcement conducts a security sweep of the aircraft."

The Denver Police Department said, "the plane has been cleared and the threat appears to be unfounded at this time," and referred further questions to the FBI, which is now leading the investigation into the potential threat.

Just after 3:30 p.m., FBI Denver reiterated that there was no evidence of a credible threat, but didn't offer any other details about the investigation.

Passengers who evacuated the plane expressed confusion and, in some cases, fear.

"I wasn't too concerned, but you could definitely feel there was a little bit of panic. I would say everybody on the plane remained pretty calm, given the circumstances," Roy Taylor told CBS News Colorado from the airport's baggage claim area.

Taylor said he got a sense that something was off based on the crew's behavior, and then was surprised by how the plane had landed.

"Something happened that definitely spooked the pilot and the crew, and I could tell that something had happened because when they landed, the plane stopped really fast, and then kind of like a hard right, and I knew the airport terminal was on the left," he said. "I knew it was a bomb threat because the first person that came on the plane was an explosives technician."

Passengers interviewed by CBS Colorado all said the flight crew and first responders wouldn't say anything about the nature of the incident. Taylor said the pilot described it simply as "a security situation."

Another passenger on the plane, Kelsey Hines, said she learned most of what she knew about the incident from CBS Colorado's coverage.

"Before reading the article on CBS, we thought there was a bomb on board. There was bomb people, bomb dogs, the whole shebang," she said. "I know there's protocols, I know there's things they can and can't say, but just giving us a heads up about what the process was would've been nice so we weren't all concerned as to what was going on."