The state’s top cannabis regulator is calling on the state’s senior U.S. senator to help find more banking solutions for the adult-use pot industry.

Steven Hoffman, chairman of the state’s Cannabis Control Commission, said on Boston Herald Radio earlier this week that he is asking Sen. Elizabeth Warren to help find federal solutions that will encourage more financial institutions to bank recreational marijuana businesses.

Retail pot sales in Massachusetts began in November. Since that time, Gardner-based GFA Credit Union and Swansea-based BayCoast Bank have been the only two publicly-known financial institutions saying they would bank recreational marijuana.

Both institutions are somewhat restricted in their ability to serve the entire market, with GFA Credit Union limited to serve only businesses within its field of membership, and BayCoast Bank saying it is not currently working with any facility that is strictly selling recreational marijuana.

In addition, BayCoast Bank only has about $1.63 billion in assets and Gardner only about $500 million, making both too small to bank the whole industry.

“Banking [marijuana businesses] is a challenge, and I’ve been very public about that for a long time,” Hoffman said on Boston Herald Radio. “Because of the federal prohibition, banks for the most part have been leery about entering into this business to support licensees.”

That challenge was seen recently when the $4.85 billion asset Century Bank, the first publicly known financial institution in Massachusetts to bank medical marijuana businesses, recently told the Boston Business Journal that it had no intention to start banking recreational businesses.

The Medford-based bank’s CEO Barry Sloane cited current federal law as his primary reason.

The Senate passed the Rohrabacher-Blumenauer Amendment in 2014 that bars the U.S. Department of Justice from using taxpayer dollars to prevent states from implementing medical marijuana laws. But the law doesn’t address recreational marijuana.

Even with medical marijuana, Century Bank has not been immune from litigation. The bank was named a co-defendant in a federal RICO lawsuit earlier this year for serving the Georgetown-based medical facility Healthy Pharms.

Working alongside Sen. Cory Gardner (R-Colorado), Warren filed a bill earlier this year that would let states choose how to regulate marijuana without having to worry about the federal government intervening.

The bill seemed to have solid support, but Hoffman said he was unsure if it would be enough to make someone like Sloane and Century Bank feel safe enough to begin banking recreational marijuana.

“It won’t fix everything and would still have some challenges for banking, but it will I think make things easier,” he said. “The thing that would make it very straightforward is taking it off schedule one.”