Amid anxiety over the 10th anniversary of the Sept. 11 attacks, police detained three passengers at Detroit's Metropolitan Airport Sunday after the crew reported suspicious activity by passengers.

Frontier Airlines Flight 623 from Denver, which had 116 passengers on board, landed without incident in Detroit after the crew reported that two people were spending an extraordinary long time in a bathroom, according to Frontier spokesman Peter Kowalchuck.

The North American Aerospace Defense Command deployed two F-16 jets to shadow the flight until it landed safely, airline and federal officials report.

The suspicious behavior ended up being two people who were stowing away in the lavatory to make out mid-flight, law enforcement officials told ABC News.

It is unclear as to why the additional passenger was taken into custody. Kowalchuck said in a statement that no arrests were made.

There were multiple airline scares on Sunday in relation to the anniversary of the 9/11 attacks. In another incident, a pair of fighter jets were scrambled to escort an American Airlines jet from Los Angeles into New York's John F. Kennedy International Airport after the pilot also became concerned by passengers' frequent trips to the bathroom.

Federal air marshals who were onboard the flight were able to resolve the situation, ABC reports.

Meanwhile, in Kansas City, Mo., The Associated Press reports security agents detained a man and shut down a terminal at the main airport Sunday after detecting suspicious items inside his carry-on bag. The items later tested negative for explosive materials.