LOS ANGELES—Federal officials have launched a fresh offensive against California's medical-marijuana industry—the nation's largest—threatening to prosecute those associated with pot dispensaries from San Francisco to San Diego.

The crackdown is the start of a coordinated effort among prosecutors in Los Angeles, Sacramento, San Diego and San Francisco. Federal officials, in an advisory, said they would reveal the scope of their plan "targeting [the] commercial marijuana industry" on Friday.

Some...