BOGOTA, Colombia — Forget bulk cash. Heavy and hard to hide, it’s simply not the most convenient cross-border conveyance for a 21st-century money launderer.

A safer and increasingly attractive alternative for today’s criminal is electronic cash loaded on what are called stored-value or prepaid cards. Getting them doesn’t require a bank account, and many types can be used anonymously.

U.S. crimefighters consider the cards a burgeoning threat that regulators haven’t adequately addressed.

In the past year, said John Tobon, a senior U.S. Immigration and Customs Enforcement agent, the cards have become the preferred means of paying couriers who transport illicit drugs across the U.S.

No one knows how big a role the cards play in moving the more than $20 billion in drug earnings that U.S. authorities estimate crosses from the U.S. to Mexico annually. Yet while anyone crossing that border with $10,000 or more in cash must declare it, prepaid cards are legally exempt.

“Law enforcement loses lives all over the world trying to keep (major criminals) unbanked, and these prepaid cards are offering them a great alternative to sneak into our financial system,” said Tobon.

‘A whole new way of doing business’

Visually, the cards are barely distinguishable from credit or debit cards and the most versatile let users reload them remotely without having to reveal their identity, using cash, moneygrams, PayPal and other online payment services.

Some cards can process tens of thousands of dollars a month. Just load them up in Connecticut or Texas with, say, the proceeds of cocaine sales and collect the cash in local currency from an ATM in Medellin, Colombia, or elsewhere in Latin America.

“I’m not so sure we have a sophisticated understanding of how to deal with this,” said Richard Stana, who oversaw a report on prepaid access for the General Accounting Office, the U.S. Congress’ research arm. “It’s just a whole new way of doing business.”

In one of the first cases to clue law enforcement to the threat, a Dallas-based company called Virtual Money Inc. provided the cards to crews who helped Colombian drug traffickers move at least $7 million to Medellin during three months in 2006, prosecutors say.

The money moved digitally, as most legitimate capital travels these days, but bypassed bank accounts, making its digital footprint harder to detect. Virtual Money allegedly violated U.S. banking law by not reporting transfers of above $10,000 or other activity suggesting illegal money movement.

David Zapp, a New York attorney for a defendant sentenced to 45 months in prison in the case, said his client was a small player in a scheme in which cards he was aware of had relatively low load limits of $1,000.

Catching on

The trick was volume — and the ability to replenish the cards. Some launderers probably had 400-500 cards, Zapp said.

State and local police in the U.S. are only just waking up to the cards, so ICE created an explanatory pamphlet it is distributing far beyond Customs and Border Patrol agents.

“We’re involved in a case much larger than Virtual Money,” said Paul Campo, chief of the DEA’s financial crimes unit. It is in the Southwest, he said, adding that the DEA also has active cases in New England and the state of Georgia.

While offering more options to money launderers, prepaid cards also are changing the way ordinary law-abiding citizens and businesses and even governments handle money. Wal-Mart uses them to distribute payrolls, and U.S. government agencies to deliver benefits such as food assistance. Migrant workers use them to send money home.

In the U.S. alone, an estimated $107 billion moved on branded prepaid cards last year, according to Aite Group, a financial research firm. Globally, the Boston Consulting Group forecasts, transactions with reloadable prepaid cards will reach $840 billion a year by 2017.