Unlike telephone lines and e-mail communications, the SWIFT network cannot be easily tapped. It uses secure log-ins and state-of-the-art encryption technology to prevent intercepted messages from being deciphered. "It is arguably the most secure network on the planet," said the former SWIFT executive who spoke on condition of anonymity. "This thing is locked down like Fort Knox."
SWIFT said it was responding to compulsory subpoenas and negotiated with U.S. officials to narrow them and to establish protections for the privacy of its customers. SWIFT also said it has never given U.S. authorities direct access to its network.
"Our fundamental principle has been to preserve the confidentiality of our users' data while complying with the lawful obligations in countries where we operate," SWIFT said in its statement.
Current and former U.S. officials familiar with the SWIFT program described it as one of the most valuable weapons in the financial war on terrorism, but declined to provide even anecdotal evidence of its successes.
A former high-ranking CIA officer said it has been a success, and another official said it has allowed U.S. counterterrorism officials to follow a tremendous number of leads. CIA officials pursue leads overseas, and the FBI and other agencies pursue leads in the United States, where the CIA is prohibited from operating.
Officials said the program is relied upon especially heavily when intelligence chatter from phone and e-mail intercepts suggested an imminent attack, conveying real-time intelligence for counterterrorism operations.
The former SWIFT executive said much can be learned from network messages, which require an actual name and address of both the sender and recipient, unlike phone calls and e-mails, in which terrorist operatives can easily disguise their identities.
"There is a good deal of detail in there," he said.
As the global war on terrorism has succeeded in taking out some senior terrorists and their financiers, particularly within Al Qaeda, the organization and its many affiliates have sought to move to hidden locations and to transfer their money through proxies such as charities, aid organizations and corporate fronts.
The officials said the SWIFT information can be used in "link analysis." That technique allows analysts to identify any person with whom a suspected terrorist had financial dealings -- even those with no connection to terrorism. That information is then mapped and analyzed to detect patterns, shifts in strategy, specific "hotspot" accounts, and locations that have become new havens for terrorist activity.
The SWIFT program is just one of the Bush administration's post-Sept. 11 initiatives to collect intelligence that could include information on U.S. residents.
The National Security Agency, which can intercept communications around the world, is eavesdropping on the telephone calls and e-mails of some U.S. residents without obtaining warrants. And it has been accused of asking telecommunications companies to help create a database of the phone-call records of almost all Americans.
The Justice Department also has asked Internet companies to keep records of the websites customers visit and the people they e-mail for two years, rather than days or weeks, which would greatly expand the government's ability to track online activity.
Numerous lawsuits have been filed against the government and phone companies, challenging the NSA efforts. The government has asked courts to throw them out, invoking the "state secrets" privilege and arguing that trials would compromise national security. The NSA's interception of telephone calls also has been criticized for lacking an independent review process to ensure that the information is not abused.
The SWIFT program raises similar concerns, some critics say.
Privacy advocates have questioned "link analysis" because it can drag in innocent people who have routine financial dealings with terrorist suspects.
And no outside governmental oversight body, such as the Foreign Intelligence Surveillance Court or a grand jury, monitors the subpoenas served on SWIFT.
Levey said the program is subject to "robust" checks and balances designed to prevent misuse of the data. He noted that requests to access the data are reviewed by Treasury's assistant secretary for intelligence; that analysts can only access the data for terrorism-related searches; and that records are kept of each search and are reviewed by an outside auditor for compliance.
Levey said there had been one instance of abuse in which an analyst had conducted a search that did not meet the terrorist-related criteria. The analyst was subsequently denied access to the database, he said.
During the last five years, SWIFT officials have raised concerns about the scope of the program, particularly at the outset, when it was handing over virtually its entire database. The amount of data handed over each month has been winnowed down.