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Morgan Fox, spokesman for the Marijuana Policy Project, said his organization is drafting a letter asking the ATF to use its discretion “to be more compassionate toward medical-marijuana patients.”

“The ATF said their hands were tied, but that’s not the case,” Mr. Fox said. “This is just another way to single out and harass medical-marijuana patients.”

Montana voters legalized medical marijuana in 2004, but its use exploded after the Justice Department’s 2009 memorandum saying federal authorities would not target marijuana users as long as they comply with state law, Montana Justice Department spokesman John Doran said.

“We went from 3,000 to 30,000 medical-marijuana cardholders in the space of about a year,” he said. “We also saw an increase in the number of types of businesses.”

Those included so-called “cannibis caravans,” which traveled throughout the state signing up people for medical-marijuana cards. Often there was no doctor on site, but rather a doctor from another state who spoke with patients via computer, Mr. Doran said.

The Montana legislature cracked down on the industry earlier this year with a bill that repealed the voter-passed initiative and replaced it with tighter restrictions. In response, advocates gathered enough signatures to place the more liberal version on the November 2012 ballot.

“States, including Montana, have acted to address problems and explore workable solutions,” Mr. Bullock said. “In doing so, however, we also face issues that are, candidly, created or exacerbated by federal actions and policies that do not always reflect the kind of careful approach and appropriate accommodation that should be accorded the states.”