Several American newspapers have revealed that bank wire transfers were being monitored for terrorist activity. Some Islamic terrorists apparently suspected as much. It's not public knowledge about how they knew this. They might have been tipped off by prior knowledge of the banking industry, or tips from people in the criminal underground who were aware of the risks inherent with using wire transfers. The risks are there because of pattern analysis. This is a collection of statistical tools that can reveal patterns in wire transfers. By using a database of organizations and individuals, known or suspected of terrorist involvement, pattern analysis will spotlight those who are involved in terrorist activities, or drug dealing, or whatever you are looking for. The wire transfer system that was being monitored moved over 11 million payments a day. Several terrorists are known to have been caught because of the wire transfer monitoring. However, because the media has made it widely known that this technique is being used, those terrorists who were using wire transfers, will switch to other forms of moving money. Actually, many terrorists have been doing that all along. Sending a courier with cash is slower and more dangerous, but it sure beats transmitting your activities to the police.
Alternative methods of transferring money are probably also being monitored, and will still be useful until some enterprising investigative journalist reveals that information to the terrorists.