Mrs Maldonado and her husband Gary were reportedly seen in security footage boarding the coach in the Mexican city of Los Mochis, on their way home to Phoenix, Arizona.

BBC: Yanira Maldonado freed from Mexican jail

Security has also been ramped up at U.K. airports and at Schiphol Airport in Amsterdam, where the suspect, Umar Farouk Abdulmutallab, went through "normal security procedures" before boarding the flight, according to security officials.

CNN: Security ramped up at airports in U.S., UK, Amsterdam

It could undermine the US's PreCheck system which randomly decides which frequent fliers can skip part of the pre-boarding security process.

BBC: Boarding pass barcodes 'can be read by smartphones'