Cannabis stocks plunged on Thursday when Attorney General Jeff Sessions rescinded an Obama-era policy that helped states legalize marijuana with little to no law enforcement interference by the federal government, according to multiple reports. Stocks started slumping soon after the Associated Press first reported the rollback.

Why it matters: It's unclear how Sessions' decision would impact marijuana sales, which is legal on the state level in eight states and Washington, D.C., for recreational use and in 29 states, including the District, for medical purposes. The Justice Department's move now means that U.S. attorneys decide how to enforce federal laws banning marijuana use.

The backdrop: Sessions' decision comes the same week California legalized retail sales, making it the largest legal pot market in the country. Cannabis industry companies have gained nearly $2 billion in value since the state started selling recreational marijuana legally on Monday, according to Fortune.

Go deeper: Sessions cracks down on marijuana legalization