Suspicious materials confiscated by security officers at four different U.S. airports have gotten the attention of the Transportation Security Administration (TSA).
The agency, operating under the assumption that terrorists often stage rehearsals before they attack, has issued an alert to air marshals, law enforcement groups, and its own officers.
At the same time, the TSA says there is no credible or imminent threat. It has posted more than 90 previous unclassified bulletins targeted for enforcement agents.
The security level at all American airports remains at orange, the second-highest, while it remains yellow for the rest of the country.
The July 20 private TSA bulletin states that agents in San Diego, Milwaukee, Houston, and Baltimore seized items that might be connected to a terrorist plot in progress.
Items taken included wires, switches, pipes and tubes, cellphone components, and even thick, potentially-masking substances such as blocks of cheese.
The men and women who carried the items were not linked to any criminal groups but allegedly gave "suspicious" explanations when asked about the confiscated contraband.
The TSA also reports that there has been an increase in the number of incidents involving materials that were unusual in nature.
Security officials say that such major incidents as the Twin Towers attacks, Madrid train bombing, and London transit system attack were all preceded by practice runs.
