*Update: ABC News now reports that "concerns about a possible terror 'dry run' involving the two eased" and that they were released from custody without being charged.

The arrest of two Yemeni men Sunday night after they flew from Chicago to Amsterdam broke up what "was almost certainly a dry run, a test," a senior law enforcement official told ABC News.

Ahmed Mohamed Nasser al-Soofi and Hezam al-Murisi have not been charged with any crime, but are being held by Dutch authorities on suspicion they conspired to commit a terrorist act. Security screeners found cellphones taped to watches and to a bottle of Pepto-Bismal inside Soofi's luggage.

That bag went to Dulles Airport outside Washington, where Soofi originally was supposed to fly. But he changed itineraries and flew from Chicago to Amsterdam. Officials say the bag had been determined to be harmless and no passengers were endangered.

Soofi and Murisi's ultimate destination was said to be the Yemeni capital Sanaa. Soofi was questioned by airport security officers in Alabama.

Air marshals kept an eye on the men during the flight, ABC reported. The network cited an unnamed senior U.S. official saying the number of marshals in the sky has been increased recently due to a heightened state concern over possible airline hijackings.

An Associated Press report casts doubt on the ABC report, saying the men are unlikely to be charged after the FBI's initial investigation minimized the likelihood it was a test run.

A "dry run" to test security procedures is not something new, however. The 9/11 hijackers were seen in Logan airport and on at least one previous flight of one of the planes they ultimately commandeered. And the 9/11 Commission report detailed a 1999 incident in which two men were detained on an America West flight after trying to enter the cockpit. They claimed they mistook the door for the lavatory entrance.