At the request of U.S. officials, Dutch authorities arrested two men at Amsterdam's airport Monday on suspicion of staging a rehearsal for a terror attack.
The request came after the two passengers, Ahmed Mohamed Nasser al-Soofi and Hezam al-Murisi, boarded a United Airlines flight at Chicago's O'Hare International Airport.
"We have to be very careful when it comes our security of our country. O'Hare Airport is a very busy airport. I think we have to take everything seriously," Illinois Gov. Pat Quinn said. "And that's what I have to do as Governor and that's what we all have to do as American citizens."
Red flags had gone up before the men reached O'Hare.
Click play to watch Efrem Graham's report. Also, for more on the terrorism threat coming out of Yemen, click here for insight from CBN News Terrorism Analyst Erick Stakelbeck.
Airport security screeners in Birmingham, Ala. found $7,000 in cash on one of the men after his bulky clothing aroused suspicion.
Dutch authorities reportedly found a mobile phone taped to a Pepto Bismol bottle, three large knives and a box cutter in their checked luggage.
"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 U.S. Homeland Security Department released a statement saying it had informed the Dutch authorities of suspicious items in the men's luggage.
"The items were not deemed to be dangerous in and of themselves and as we share information with our international partners, Dutch authorities were notified of the suspicious items," the statement read.
According to one U.S. law enforcement official, the men's carry-on luggage contained no banned items. He also said federal air marshals were also on board the flight.
Al-Soofi had a Michigan address, but authorities are as yet unsure where the two were living, The Associated Press reported.
According to the AP, FBI agents also checked several southwest Detroit residences for men with variations of al- Soofi's name.
Ahmad Rehab, executive director of Chicago's Council on American-Islamic Relations (CAIR), advised people not to jump to conclusions.
"If it's just a case cell phones and antacid. Well, that's kind of not really good enough," Rehab said.
However, officials said their suspicions were further raised when both men boarded a different flight than their checked baggage.
Both men changed their original flights, another factor that alerted U.S. security officials.
"After 9/11, that's rule number 1 - that passengers and travelers go together, but not severed," aviation expert Joe Schwieterman explained. "It creates opportunity for sort of walk-away terrorism."
When customs and border officials learned al-Soofi was not on the flight with his luggage, that plane was ordered to return to the gate so his luggage could be removed.
Officials said additional screening found no evidence of explosives.