Dutch police questioned Tuesday two US residents of Yemeni descent arrested after flying into the Netherlands from Chicago in what officials fear may have been a dry run for a terror attack.

The White House promised US authorities will conduct a “vigorous investigation” into the incident, which officials don’t exclude may have simply been a misunderstanding.

US airport security screeners found suspicious-looking items in the men’s checked luggage late Sunday before they flew out of Chicago, including a cellphone taped to a medicine bottle, three cellphones taped together, watches taped together, and box cutters and knives, according to US news reports.

One of the men, a 48-year-old, was also carrying 7,000 dollars in cash.

US officials notified Dutch authorities the men’s luggage included “suspicious items,” the Department of Homeland Security said.