As marijuana shops sprout in states that legalized the drug, they face a stumbling block: lack of access to the kind of routine banking services other businesses take for granted.

US Senator Elizabeth Warren, a Massachusetts Democrat, is leading an effort to make sure vendors working with legal marijuana businesses, from chemists who test marijuana to firms that provide security, don’t have their banking services taken away.

It’s part of a wider effort to bring the $7 billion marijuana industry in from a fiscal limbo Warren said forces many shops to rely on cash, making them tempting targets for criminals.

After voters in Warren’s home state approved a November ballot question to legalize recreational use of pot, she joined nine other senators in sending a letter to the Financial Crimes Enforcement Network, calling on it to issue guidance to help banks provide services to marijuana shop vendors.

Twenty-eight states have legalized marijuana for medicinal or recreational use.

Warren said there are benefits to letting marijuana-based businesses move away from a cash-only model.

‘‘You make sure that people are really paying their taxes . . . And it’s just a plain old safety issue. You don’t want people walking in with guns and masks and saying, ‘Give me all your cash.’ ’’

Two years ago, the Department of the Treasury gave banks permission to do business with legal marijuana entities under some conditions. Since then, the number of banks and credit unions willing to handle pot money has risen from 51 in 2014 to 301 in 2016.

Warren, however, said fewer than 3 percent of the nation’s 11,954 federally regulated banks and credit unions are serving the cannabis industry.

Taylor West, deputy director of the National Cannabis Industry Association, a trade organization for 1,100 marijuana businesses nationwide, said access to banking remains a top concern.

‘‘What the industry needs is a sustainable solution that services the entire industry instead of tinkering around the edges,’’ Taylor said.