US officials have said they do not believe two Yemeni men arrested in Amsterdam after flying from Chicago were planning a terror attack.
The men were detained at Schiphol airport after they were found to have checked luggage onto a US internal flight but not boarded it.
Airport authorities had earlier found "suspicious items" in one of the men's luggage but had cleared it to fly.
The two men are being held by the Dutch authorities.
They are being held "on suspicion of a conspiracy to a terrorist criminal act". The Dutch authorities have until Thursday to charge or release them.
"We are taking it seriously. Otherwise we would not have arrested them," said Theo D'Anjou, a spokesman for the Dutch national prosecutor's office.
He said an investigation was under way "to see whether we can charge them, and if we can charge them, with what".
But in a statement, the US Department of Homeland Security urged caution over the incident.
"This matter is under investigation but as of right now, these two passengers have not been charged with any crime in the United States and we caution you against jumping to any conclusions," it said.
The two men, who had both been travelling to the Yemeni capital Sanaa, were arrested on arrival at Amsterdam's Schiphol airport on Monday morning.
They had checked luggage onto an internal flight in the US that they did not then take. Officials in the US say it appears they missed the flight and were re-routed by American Airlines to travel via Amsterdam.
That flight, from Chicago's O'Hare to Washington Dulles International Airport, was called back once it was found they were not on board.
US officials believe the two men did not know each other and were not travelling together.
One of the men had earlier been stopped by airport officials on a connecting flight from Birmingham, Alabama.
He was found to be carrying $7,000 in cash and when his luggage was searched, officials found a mobile phone strapped to a medicine bottle, as well as knives and watches.
The luggage was cleared for the flight after it was not judged to be a threat but there was speculation that the chain of events could have been a dry-run for a terror attack, testing the US flight security operations.
Customs authorities said it was not uncommon for people travelling to the countries like Yemen to be carrying large amounts of cash and that valuable items are often found bundled together.
Klaas-Arjen Krikke, a lawyer representing one of the men, said his client had recently resigned his job and was travelling to his home in Yemen. He also criticised the way information about the incident had been released.
"My client has already been condemned by a large section of the public via the media," Reuters quoted him as saying.
The BBC's Christian Fraser in Amsterdam says that whatever the investigation into the incident eventually concludes, serious questions will be asked about the airport security in the US.
White House spokesman Robert Gibbs said the man had not been on any terror watchlists.
But he told CNN the US would conduct "a vigorous investigation to see if we can match up any of the circumstances that were involved with any intelligence that we might have".