Despite security concerns surrounding them, Ahmed Mohamed Nasser al Soofi and Hezem al Murisi were allowed to board United Airlines flight 908 at O'Hare.
The arrests come at a time of heightened alert just days before the ninth anniversary of the Sept. 11, 2001 terror attacks.
Airport screeners in Birmingham, Alabama first stopped al Soofi and sent him to additional screening because of his "bulky clothing. A check of his luggage turned up large amounts of cash and cell phones taped together and to a Pepto-Bismol bottle. U.S. Officials also found a knife and box cutters in one of the men's luggage, but Transportation Security Administration screeners determined that the items didn't pose a threat. No explosives were found.
One official said that it may sound strange, but travelers heading overseas often tape things together in their luggage for fear that items will be damaged in transit.
Once arriving in Chicago from Birmingham, officials said al Soofi checked his luggage on a flight to Washington Dulles airport but did not board the flight himself. Instead, he joined al Murisi for the flight to Amsterdam. On international flights, passengers and their luggage must be headed toward the same destination, according to U.S. policy.
The men were arrested at Amsterdam's Schiphol Airport on Monday. One U.S. official said it appears the Dutch were not asked to hold either man but were instead notified of the odd behavior and apparently decided on their own to detain them.
While the two men were picked up by the Dutch, al Murisi is now thought to be uninvolved and was simply coincidentally sitting next to al Soofi, who's from Detroit.
Also on board the flight were three federal air marshals, the Department of Homeland Security said.
Neither al Soofi nor al Murisi were previously of concern to law enforcement, a law enforcement official said, nor is there anything to suggest they had ill intent.
The FBI and the Department of Homeland Security say the concern over the incident is low.