Hawaii’s fledgling medical marijuana industry was dealt a blow Thursday with the Trump administration rescinding a policy that kept federal authorities from prosecuting people for legal pot sales.

U.S. Attorney General Jeff Sessions is giving federal prosecutors the power to decide which cases to prosecute when state rules are in conflict with federal laws banning the use of cannabis.

The move could slow the rapid spread of legal marijuana use in Hawaii and nationwide. California on Monday became the seventh state to legalize the sale of recreational pot. Hawaii is one of 29 states with legal medical marijuana programs.

“I can’t say with any degree of certainty that it (Sessions’ new policy) won’t affect us. It’s frankly scary,” said Richard Ha, CEO of Hawaii island dispensary Lau Ola, which is preparing to open in Hilo and Kona this summer. “We don’t think we’re crooks. We think we’re doing a good service for our community.”

It is not clear how the change might affect states that legalized medical marijuana, but a congressional amendment blocks the Justice Department from interfering with those programs. Justice officials said they would follow the law but would not preclude the possibility of medical-marijuana- related prosecutions. The feds also have the authority to sue states on the grounds that state laws regulating pot are unconstitutional.

“What a complete waste of time,” Congresswoman Colleen Hanabusa said in a statement. “Times have changed. More than half the population of this country lives in a place where marijuana is legal for recreational or medicinal use. We cannot turn back the clock. What we cannot do, is throw uncertainty into a growing market space and leave thousands of legal users and patients in limbo.”

State Attorney General Doug Chin told the Honolulu Star-Advertiser the federal action sets back efforts by the Legislature and administration to develop a safe and robust medical cannabis dispensary system in Hawaii.

“The people who may end up being hurt the most are state-licensed medical cannabis patients,” Chin said. “The state Attorney General’s Office will defend its state laws.”

There were nearly 20,000 medical marijuana cardholders in Hawaii as of Dec. 31, up from 15,334 and 13,150 at the same time in 2016 and 2015. Two dispensaries are operating on Oahu and two on Maui, with more preparing to open.

Teri Gorman, spokeswoman for Hawaii’s first dispensary, Maui Grown Therapies, said, “The industry could be beat with a single tweet. The concern is the unpredictability of this administration.”

The nation’s legal cannabis industry is projected to generate $16 billion in 2017 and as much as $40 billion by 2021 in economic impact, according to the The Arcview Group, which estimates more than $1 billion in wholesale, excise and sales taxes were taken into state treasuries in 2016. That number is expected to grow to more than $1.4 billion in 2017 and nearly $2.8 billion by 2021, not including additional state and local general sales taxes.

The Obama administration in 2013 announced it would not stand in the way of states that legalize marijuana, as long as officials kept it out of the hands of criminal gangs and children and places it was outlawed. That memo, written by then-Deputy Attorney General James Cole, had cleared up some of the uncertainty about how the federal government would respond as states began allowing sales for recreational and medical purposes.

“There is now greater uncertainty as to how federal agencies will treat state adult use cannabis programs. There may also be a slowing effect for patient demand and state plans for expanding adult use cannabis programs,” Helen Cho, spokeswoman for Oahu dispensary Aloha Green Apothecary, wrote in a memo to employees. “Now more than ever it is vital for Aloha Green Apothecary to show the patients, the public, and the government that regulated cannabis dispensaries can work.”

State Sen. Will Espero, a cannabis advocate, said he doesn’t expect the policy change to affect Hawaii’s growing program.

“It’s unfortunate that this memo was issued, especially when you see right now 30 jurisdictions nationwide that have medical marijuana programs,” Espero said. “Obviously, attitudes and thinking of people have changed. Sessions’ action just brings fear and apprehension when it is not necessary. We shouldn’t feel too threatened at this time. That train is moving, and it’s going to be difficult to stop.”

Sessions’ statement brought back memories of Hawaii’s “Green Harvest,” a controversial marijuana eradication program started decades ago that involved choppers that would fly over fields to search for illegal pakalolo, primarily in the state’s rural communities.

“Everybody on the Big Island remembers that,” Ha said. “They would fly and then confiscate the plants they see.”

The Associated Press contributed to this report.