As they hit the trail of the terrorists behind Tuesday's outrage, investigators are poring over credit card receipts in search of financial signs pointing to where they came from and who paid their way here, sources say.

In what could be the first major break in the terror probe, law enforcement officials have uncovered the purchase with a single credit card of seven tickets on the two flights hijacked from Logan International Airport, sources say.

The credit card purchase led to the dramatic apprehension yesterday afternoon of three people at the Westin Hotel, sources say.

For investigators of Tuesday's terror attack, examining the suspicious credit card will most likely be just the first step in a long and winding financial trail, experts say.

Uncovering the money trail, in turn, could eventually enable the U.S. government to locate and help seize accounts and assets used to finance the worst attack on the U.S. since Pearl Harbor.

But there is no guarantee that investigators will be able to totally shut off the flow of hot money, with terrorists becoming increasingly sophisticated in covering their financial tracks.

"I would say lots of luck," said Edith Flynn, a professor emeritus of criminal justice at Northeastern University. "The flow of money is extremely well hidden. There are so many other lines and sidelines."

For starters, law enforcement officials are expected to intensively mine the credit card lead, experts say. Customarily, credit card issuers will require a consumer to provide them with a bank account number, said Jack Blum, a Washington lawyer and former special counsel to the Senate Foreign Relations Committee.

"That's really when you start finding out where the money came from," said Blum, who was once an aide to Sen. John F. Kerry (D- Mass.).

Oddly enough, obtaining a credit card is not that hard for terrorists, and is preferable to paying cash, which would raise suspicions, Blum noted. False passports or other forms of ID are easy to get in many places overseas, he added, and those could be used to obtain a credit card.

Law enforcement officials will also eye other possible sources of financial support for the terrorists, which could include a range of illegal dealings.

The investigation of the 1992 World Trade Center bombing uncovered extensive illegal sales of counterfeit copies of watches and other consumer products, said Michael Kessler, a New York-based specialist in forensic accounting who was a consultant on the earlier Trade Center bombing probe.

"You can't carry this off if you are a beggar in the street. Someone had to fund this operation and fund it big time," Kessler said.

However, tracking down and seizing the big money - the intricate web of accounts and funds ultimately behind the massive terrorist attack - could prove more elusive, experts say.

Suspected terrorist mastermind Osama bin Laden, one of the main suspects in Tuesday's devastating attack, has been the focus of law enforcement scrutiny since at least the early 1990s. In fact, bin Laden was believed to be behind the 1992 Trade Center bombing.

While bin Laden's assets may still flow to fanatics bent on launching ever more dramatic attacks, it may not be the fault of the U.S. government.

John D. Moore, a former U.S. State Department terrorism expert who is now a Tufts University graduate student, said terrorists have become skilled in evading financial detection.

Middle Eastern terror groups are known to employ ancient banking practices involving couriers traveling from city to city with paper vouchers. A system based on trust and personal connections, such money networks can be very hard to track.

Still, Tamar Frankel, a Boston University law professor and an expert on financial institutions and corporate law, believes a coordinated attack on the sources of terrorist financing by the U.S. and countries around the world could ultimately prove less costly than massive military retaliation.

The key to such an approach would be for the U.S. to convince countries that may be harboring - and potentially profiting from - terrorist bank accounts to participate in such a crackdown.

South Africa and various islands in the Caribbean are known to be hot spots for money laundering.

"If all the countries were to get together and say we will simply not allow this money to be used . . . in the last analysis it's cheaper, it's cleaner, and our people will not get killed," Frankel said.