To banks, the pre-eminence of federal law has been a powerful deterrent to allowing pot businesses to set up accounts. In fact, Don Childears, chief executive of the Colorado Bankers Association, said his reading of the federal law was that “the very receipt of a deposit is the definition of money laundering.” His train of logic: Marijuana is illegal at the federal level; banks that take money from illegal drug operations are guilty of money laundering; therefore, the banks that take pot money face serious criminal and civil liability.

On Feb. 14, 2014, the Justice Department and the Treasury introduced a wrinkle. They each sent out guidelines that seemed to crack open the door for banks to engage with marijuana businesses. The Justice guidelines, known as the “Cole Memo,” didn’t foreclose the possibility that banks taking marijuana money could be prosecuted for financial crimes, but directed prosecutors to go after significant cases, which it defined using eight priorities. For example, a bank that provided services to a marijuana business that distributed to minors or sold in states where the drug was illegal or used gun violence might be subject to prosecution, but when such aggravating factors were absent, prosecution “may not be appropriate,” the guidelines said.

Under the Treasury guidelines, banks are urged to file “suspicious activity reports” when a new pot business opens or closes an account or when such businesses exhibit activities that violate the Cole guidelines. Treasury characterized the efforts in part as a way to get money into the banking system, where it — and its actors — could be more easily tracked. In sum, the guidance was advertised by many as a “green light” to banks.

Mr. Childears of the Colorado Bankers Association doesn’t see it that way. “They were a yellow light at best,” he said of the guidelines. In fact, he argued, “They raised the liability for the banks.” The costs of filing suspicious activity reports, he said, are considerable and raise all kinds of questions. For example, he asked, what if such a report actually becomes an admission that the bank is participating in an illegal enterprise?

Plus, Mr. Childears said, the guidelines are not law. In other words, they don’t preclude a federal prosecutor or state attorney general from going after a bank. In fact, two state attorneys general, from Nebraska and Oklahoma, sued the state of Colorado in December, asking that the Colorado law that legalized pot be struck down because it violates federal law and “creates a dangerous gap in the federal drug control program.”

Most banks are wary of stepping into this mire, Mr. Childears said, estimating that perhaps 5 percent of Colorado’s marijuana businesses use a financial institution (others put the figure higher, but still below half the pot business in the state).