U.S. Air Force jets were scrambled Sunday to escort two passenger airline flights because of security scares as Americans marked the 10th anniversary of the Sept. 11 attacks.

Security concerns also led officials to close parts of the airport in Kansas City, Mo., on Sunday for hours and question a passenger after screeners at a checkpoint raised suspicions about the contents of a bag.

Two F-16 fighter jets escorted an American Airlines flight to a safe landing in New York after a report of suspicious activity on the flight from Los Angeles.

The Transportation Security Administration said it was notified of passengers behaving suspiciously on American Airlines Flight 34 from Los Angeles International Airport to New York's John F. Kennedy International Airport.

Out of an abundance of caution, NORAD (North American Aerospace Defense Command) scrambled two F-16 jets to shadow the flight until it landed safely at JFK, the agency said in a statement.

The Boeing 767-200 landed and the passengers disembarked without incident, the airline said.

A spokeswoman for American said the crew did not perceive a threat.

Our captain and flight attendants did not see that there was any security issue, Andrea Huguely told Reuters, adding that the crew and airline did not ask for assistance.

Another American Airlines spokesman confirmed that the incident began with suspicious behavior observed in the area of the jetliner's lavatory.

In another incident on Sunday, F-16 jets were called into action to shadow Frontier Airlines Flight 623 to Detroit Metro Airport after the TSA was notified of passengers behaving suspiciously, the agency said.

Law enforcement met the flight, which was brought to a remote area of the airport, The TSA said. The plane was swept with negative findings and cleared.

The flight, which originated in Denver, landed safely in Detroit. An airline spokesman said two passengers had been seen to be spending an extraordinarily long time in the lavatory and three passengers were taken into custody.

Sandra Berchtold, a spokeswoman for the FBI in Detroit, said the three passengers were being interviewed and were not under arrest.

Federal, state and local authorities said they approached the incident reports with an abundance of caution after a warning late last week of credible but unconfirmed information that three al-Qaida operatives had snuck into the country and were planning an attack to coincide with Sunday's anniversary.

In Kansas City, the FBI and airport police bomb technicians were summoned because the passenger was uncooperative after screening agents flagged the baggage as suspicious, an FBI spokeswoman there said.

The bag was dismantled, but no explosive materials were found and all parts of the Kansas City airport reopened later.

(Reporting by JoAnne Allen and Clare Baldwin in New York, Alex Dobuzinskis in Los Angeles, Kyle Peterson and, James B. Kelleher in Chicago and Kevin Murphy in Kansas City; Editing by David Bailey)