Two airline passengers arrested after scans revealed suspicious items in their luggage may have been on a dry run for a terrorist attack, security sources have said.
The men were held at Amsterdam airport after flying in from the US at a time of heightened alert just days before the ninth anniversary of the September 11 2001 terror attacks.
Among their luggage was a mobile phone taped to a medicine bottle and a knife and box cutters.
Ahmed Mohamed Nasser al Soofi and Hezam al Murisi arrived on a United Airlines flight from Chicago, where their decision to change their flight plans raised questions.
One of the men, believed to be from the Detroit area, is of Yemeni origin.
US officials said they are investigating whether the incident was a dry run for a future terror attack and whether the men were testing the aviation security system to see if strange items and travel patterns would raise suspicion.
The White House said the men were not on any US terror watch list.
Dutch investigators questioned two men arrested at Amsterdam's airport after US authorities found the suspicious items one of the men's checked luggage.
The pair were arrested on Monday morning at Schiphol Airport and under Dutch law can be held without charge for six days.
A US homeland security spokeswoman said once officials found suspicious items in luggage associated with two passengers on Sunday night's flight, they notified the Dutch authorities.