As the Presidential Administration works to harden domestic defenses against terrorism, some experts point to a potential vulnerability from thousands of flights that pass over the United States each week. Although the United States regulates overflights, the cargo aboard them is not screened to federal standards and passenger lists are not matched to names on the terrorist watch list maintained by the Transportation Security Administration (TSA). The TSA said other countries Ã¢??have their own cargo security protocols that apply to those aircraft.Ã¢?? The TSA has not implemented the new Secure Flight program to scrutinize passengers boarding overflights. That behind-the-scenes operation is designed to ferret out potential terrorists through a process that begins with airlines collecting detailed information when someone buys a ticket. Security experts are divided about the severity of the risk. Scanning all the cargo that flies over the country Ã¢??is totally unrealistic,Ã¢?? said the director of the Center for Transportation and Logistics at Massachusetts Institute of Technology. Ã¢??We have tens of millions of packages flying almost every night. We canÃ¢??t stop the huge flow of packages from all over the world. There has to be a balance between acceptable risk and the economy.Ã¢?? But a longtime U.S. intelligence operative who teaches counterterroism courses at Embry-Riddle Aeronautical University in Arizona said a terrorist could Ã¢??explode a plane with a dirty bomb or a biological weapon or an actual nuclear weapon on board, and that material will spread wherever it crashes.Ã¢??