The U.S. government has opened a new line of business for America's biggest banks, and for once they don't want it. Little wonder: It's cash from legalized marijuana.

The financial-crimes arm of the Treasury Department is making it easier to deposit the fledgling industry's growing revenue, at last count nearly $3 billion a year and almost all in cash. The government wants to tax the revenue and keep it away from organized crime. And it figures that banks with strong compliance departments can best help it track the money.

At the same time, federal bank regulators have remained silent on the issue, raising the specter that banks could run afoul of federal drug laws if they accept the cash.

That's left the banking industry dazed and confused about what to do even as legal marijuana sellers in 23 states and the District of Columbia are faced with mountains of cash piling up in warehouses and basement vaults.

"More than 200 million Americans live in states where there is some form of legal marijuana," said U.S. Rep. Earl Blumenauer (D., Ore.) who is pushing a bill on marijuana taxation. "It's a disservice to these business people to deny them normal access to banking services."

It's been just more than one year since the Financial Crime Enforcement Network, also known as FinCEN, first provided instructions to banks on how they can both accept marijuana business dollars and still comply with the law. Since then, the industry has been surging. It's getting financing from investment funds, and pot icons such as Willie Nelson and the estate of Bob Marley are pitching new products. Yet few banks have opened their doors, prompting about 100 business people from the cannabis industry to descend on Washington last month to lobby Congress for greater access to banking.

The pressure couldn't come soon enough. With billions in cash from lawful sales of weed and marijuana cookies and sweets stranded outside the banking system, cash can't be monitored by banks for possible illegal activity.

Local officials in communities where marijuana is legal are also concerned that large stashes of cash in warehouses, businesses, and homes could create public safety issues, possibly leading to violent robberies or worse.

To minimize those risks, FinCEN officials, in meetings with bank executives to discuss a broad array of business activity outside the banking industry, are reminding them about the marijuana directive.

Yet at larger banks, Treasury's overtures have fallen on deaf ears. Citigroup Inc. and JPMorgan Chase & Co. say they won't provide services to businesses that engage in activity that is illegal under federal law.

KeyCorp chief executive officer Beth Mooney said that marijuana is still considered "a restrictive or prohibitive industry," and said that the bank hasn't had any conversations with regulators about the issue.

John Stumpf, CEO of Wells Fargo & Co., said marijuana's federal illegality stops his bank from even thinking about accepting the business. "That's out of my sights right now," he said.

It's a view also shared by some of the biggest regional lenders.

"You'd be hard-pressed to find a large bank that's willing to take that risk right now," said Jeff Bahl, a portfolio manager who helps oversee more than $7.9 billion at Bahl & Gaynor Inc., including bank stocks.