The first recreational pot market in the U.S. notched another marijuana first Thursday when Colorado started using marijuana taxes to fund police efforts to crack down on illegal growing operations, reports the Associated Press.

A measure signed into law by Gov. John Hickenlooper sets aside nearly $6 million a year in Colorado marijuana tax revenue to reimburse police for investigating black-market marijuana activity that authorities say has increased since the state legalized recreational marijuana in 2012.

"We don't want people to say they're trying to grow for medical purposes, or licensed recreational uses, and instead they're shipping it out of state," Hickenlooper said.

The fund was backed by police groups who complain that marijuana legalization has attracted illicit marijuana growers along with legal ones.

The bill was also backed by Colorado's nascent marijuana industry amid complaints that illegal growing operations undercut prices of pot grown legally and give legalization a bad name.

Colorado's fund is the first in any state designated to specifically combat the black market. Colorado gave law enforcement about $1.7 million last year for other marijuana-related enforcement activities, such as training officers to spot stoned drivers.