|There are conflicting theories as to what really led to the arrest of the two passengers [EPA]
Dutch authorities have arrested two men who arrived at Amsterdam airport from Chicago over reports that the pair were carrying "suspicious items" in their luggage.
US security officials alerted authorities in the Netherlands after they reportedly found a mobile phone taped to a bottle, along with watches, a box cutter and knives.
ABC News, a US television broadcaster added that one of the men was carrying $7,000 in cash.
It identified the pair as Ahmed Mohamed Nasser al-Soofi and Hezam al-Murisi, who are both reported to be of Yemeni descent.
Dutch airport police said that the men were arrested at Schiphol airport on Monday morning "at the request of the judicial police," but did not elaborate.
ABC News said the men boarded United Airlines 908 flight from Chicago to Amsterdam despite a number of security concerns, beginning in Birmingham, Alabama, where Soofi appeared to have started his journey.
Security staff at Chicago's O'Hare airport reportedly stopped him because of his "bulky clothing", and uncovered $7,000 in cash, before opening his luggage.
|The men were travelling on United Airlines Flight 908 from Chicago O'Hare to Amsterdam Schiphol [EPA]
The US homeland security department said in a statement it had informed Dutch authorities that two men flying to Amsterdam had packed "suspicious items" in their luggage.
But it said that the "items were not deemed to be dangerous in and of themselves", adding the matter was still under investigation.
A US law-enforcement official said the men did not have banned items in their carry-on luggage and that federal air marshals were on the flight.
Michael Barrett, a security and risk management expert and former strategy director of the White House's National Security Council, told Al Jazeera that the arrests were "an extremely serious discovery".
"What it shows is that there's a continued interest in aviation as the location of a threat for terrorist strikes. But what's really important here is this actually shows a strike emanating from the US," he said.
"It does appear that this potentially is a dry-run - a test attempt to see how the security system works. The good news is obviously that the security system worked and found them; the bad news is potentially there can be many more attempts just like this.
"These kinds of plots that actually originate within the US are much harder to find. We in the US have a very strict system by which we monitor the interactions of law enforcement with the general public and we're going to have to take a look at whether or not that's still going to keep us safe."
In the hours that followed the arrests in Amsterdam, there were conflicting theories about whether the episode was a misunderstanding or a genuine threat.
Several US media sources reported that Soofi, after commencing his journey in Birmingham, checked his luggage on a flight bound from Chicago for Yemen, with scheduled stops in both Washington's Dulles airport and Dubai.
Reports said the pair then boarded a flight to Amsterdam, and when officials at Washington's Dulles airport realised that Soofi was not on the same flight as his bag, they recalled the flight and removed the luggage.
But the New York Times said Soofi checked his bag onto a Yemen-bound flight from Birmingham, parting ways with his luggage in Alabama rather than Illinois.
Amid limited details about the incident, there were suggestions that it could turn out to be have been an unusual but harmless misunderstanding.