Two men from the Detroit area were arrested in Amsterdam on a flight arriving from Chicago, reportedly suspected of assessing the aviation system for a potential future terror attack.

The two men did not have prohibited items on them, a law enforcement official told Fox News, saying that although knives were found in their checked luggage, such checked items aren't prohibited. Law enforcement officials also found several non-functioning cell phones taped together, a phone taped to a Pepto Bismol bottle and a knife and box cutter in checked luggage connected with the men.

"Suspicious items were located in checked luggage associated with two passengers on United Flight 908 from Chicago O'Hare to Amsterdam," the Transportation Safety Administration said in a written statement. "The items were not deemed to be dangerous in and of themselves."

Law enforcement identified the men as Ahmed Mohamed Nasser al Soofi and Hezam al Murisi. Neither was on the no-fly list, nor were there any active warrants for their arrest.

Al Soofi was supposed to fly from Chicago to Washington Dulles International Airport in Virginia, and then on to Amsterdam, the official said. But when he got to Chicago, he changed his travel plans to take a direct flight from Chicago to Amsterdam. Al Murisi also changed his travel plans in Chicago to take a direct flight to Amsterdam, raising suspicion among U.S. officials. Federal Air marshals were on the flight from Chicago to Amsterdam, a law enforcement official said.

Homeland Security spokeswoman Amy Kudwa said once officials found suspicious items in luggage associated with two passengers on Sunday night's flight, they notified the Dutch authorities.

FBI agents in Chicago are assisting Dutch investigators as needed, an FBI spokesman told Fox News. As of Monday night, the Associated Press reported that FBI agents had visited the Detroit neighborhood where several addresses were found for variations of al Soofi's name, according to neighbors who declined to give their names.

ABC News, which first reported the arrests, quoted an unnamed U.S. official saying that the pair were charged by the Dutch with "preparation of a terrorist attack." One of the men initially drew suspicion when boarding a flight from Birmingham, Ala., to Chicago, but authorities allowed him to board, ABC News reports.

Another law enforcement official said, as of Monday night, the men had not been charged with anything in the U.S.

The law enforcement officials spoke on condition of anonymity to discuss the ongoing investigation and sensitive security issues.

Sources told Fox News that the men posed no immediate danger to the flight, but U.S. officials are looking into whether the men were "testing the system."

The Associated Press and Fox News' Catherine Herridge and Marla Cichowski contributed to this report.