"FedEx has a 40-year history of close cooperation with law enforcement to prevent the misuse of our networks," said Patrick Fitzgerald, the company's vice president of communications. "We have security measures in place, but we do not discuss them."

"We have many layers of security in place," said Andrew McGowan, a UPS spokesman. "However, per our corporate policy we don't discuss our security measures publicly."

Marijuana is widely grown in California under a patchwork of local laws. Martin Lee, author of "Smoke Signals," a history of marijuana, estimated that for every one pound of the drug sold through medical dispensaries in that state, nine circulate more broadly.

"I would suspect that what the postal inspectors are finding right now is a tiny fraction of what's coming into Maryland," Lee added. "A lot of people smoke marijuana and they like it."

Buck's case shows how marijuana laws are becoming more complicated as some states liberalize their approach to the drug. Buck denied ever selling marijuana and said he had a permit to grow 99 plants in California under a medical license — he uses the drug for knee pain, he said — and it was from that harvest that he sent himself a package.

Police seized $5,220 from Buck's truck when he was arrested, which authorities believed was drug proceeds, according to court records. Buck said the money was income from his real-estate business, and he is fighting in court to get it back.

Marijuana is still illegal under federal law.

Some growers are tempted to sell cross-country to get higher prices in states like Maryland where marijuana is outlawed, according to Lee. He said residents of states where marijuana hasn't been decriminalized will pay up to $300 an ounce, while prices in California have collapsed to $1,200 per pound.

Franklin, from Law Enforcement Against Prohibition, said that while some of the marijuana traveling by mail is likely traded between friends, organized drug rings also take an interest.

"You find some people who just do it for their own use, might have a relative or know someone out there," he said. And "you have people who are strictly business, and that's how they get their supply of marijuana."

Franklin said that while the highest-quality supply is grown in California, drug organizations also move lower-grade marijuana grown in Mexico into California to be mailed from inside the United States.

Baltimore Sun reporter Kevin Rector contributed to this article.

iduncan@baltsun.com

An earlier version of this article misstated James A. Buck's age. The Baltimore Sun regrets the error.