(CNN) It was a week of highs and lows for marijuana enthusiasts. Only three days after California opened its first legal weed dispensary, becoming the latest US state to trade in kosher cannabis, a massive reversal from the Justice Department has left the burgeoning industry in doubt.

Attorney General Jeff Sessions announced Thursday he was rewriting the federal government's stance on marijuana prosecution, shifting policy away from the last administration's hands-off approach to one that brings more risk to the players in states where the drug is decriminalized.

The 2013 "Cole memo" that recognized that marijuana was still illegal under federal law but gave prosecutors permission to direct resources away from enforcement in states with decriminalization laws is no more. Now, per the DOJ, federal prosecutors are to "enforce the laws enacted by Congress and to follow well-established principles when pursuing prosecutions related to marijuana activities."

How that plays out on the ground, from sunny California to snowed-in Massachusetts, is a question that has kept the phones of marijuana attorneys busy.

You may have some questions too.

Can I still buy legal weed?

"Yes you can," said Brian Vicente, a marijuana attorney based in Denver. In the six states where recreational marijuana is legalized and its sale is allowed, if you meet the requirements, there's nothing to stop you from going to a dispensary and making a purchase.

But is it risky?

"We have never historically seen the federal government prosecute people for small possession of marijuana. I think it's highly unlikely the federal government will be using their scarce federal resources to hone in on marijuana users," Vicente said.

While marijuana possession remains illegal under federal law, small-time consumers are not in the crosshairs of Thursday's announcement. Sessions, in his memo, instructs prosecutors to weigh relevant considerations like "the seriousness of the crime, the deterrent effect of criminal prosecution, and the cumulative impact of particular crimes on the community." He also mentions the DOJ's "finite resources."

Vicente, though, stresses that consumers need to know their states' marijuana laws "inside and out" -- legal purchase limits, driving under the influence and across state lines restrictions -- and warns that it's possible federal officials could prosecute violations of those.

What about business owners?

Here's where the DOJ decision could really bare its teeth. By empowering US attorneys to decide on their own whether or not to prosecute the sale of marijuana in states that have decriminalized it, risk for industry movers is greater.

The reaction from the top prosecutors in districts with legalized recreational marijuana has been mostly mild. In Colorado, the US attorney said in a statement that his office would not change its approach, which "has already been guided by" the principles outlined in Sessions' memo. Other prosecutors, from Alaska and Maine, and districts based in Seattle and Portland, said they too would continue their practices, focusing resources on combating violent crime and large-scale drug trafficking.

But still, US attorneys for the Southern District of California, encompassing San Diego, and for the District of Massachusetts, signaled a potentially stricter tact. In San Diego, US Attorney Adam Braverman said his prosecutors "will evaluate violations of (federal drug laws) in accordance with our district's federal law enforcement priorities." In Massachusetts, US Attorney Andrew Lelling said he intends "to meet with our federal partners to discuss enforcement efforts and priorities."

"If these US attorneys say they want to toe the party line and attack these marijuana companies they've got all the power of the federal government behind them, so the risk has increased," said Derek Connor, a Las Vegas-based marijuana attorney.

For many stakeholders in the marijuana industry, Thursday was a difficult day.

"I've talked to 15 clients today ranging from folks in Ohio to Florida to Denver," Vicente said. "They're pretty concerned. They're trying to peel this back and figure out what layer of risk they have. It's a bit of a scary day for those folks."

What could the prosecutors do?

Whether they'll do it or not -- with considerations of tight budgets and competing priorities -- US attorneys' offices can now begin crackdowns at various points in the billion dollar industry.

Legal authorities described potential scenarios.

"The US attorney for Boston would direct the (Drug Enforcement Administration) and say, 'listen, it is now a priority for us to go after marijuana businesses,' and they would engage in investigations or maybe even kick in the door of a marijuana dispensary clearly violating federal law and arrest them," Vicente said.

Added Connor: "It would be started like any other federal criminal investigation. I would imagine there would be some cease-and-desist letters. 'You've got a certain number of days to shut down and cease operations.'"

Confidential informants could be used to play purchasers before search warrants could be executed, Connor continued.

In any situation, the case against a legitimate distributor would be exceedingly easy, Connor said.

"In these states where everything's regulated so tightly from seed to stem, essentially you've built the feds a case for them by virtue of these federal regulations," Connor said. "You've basically handed them on evidence on a silver platter."