Americans who want to invest in Canadian marijuana firms may be involved in an illegal transaction.

The Drug Enforcement Agency could consider them in violation of the Controlled Substances Act, since marijuana is still illegal on the federal level, experts told Reuters.

The DEA is tracking the investments at the state level, in states like Colorado and Washington where the recreational use of the drug is legal, but it also revealed it is interested in the cross-border investments.

To-date there have been 23 states who have legalized marijuana.

DEA spokesman Rusty Payne told Reuters the agency is "most interested in those types of activities."

Upon this revelation, shares in some of the Canadian medical marijuana firms fell sharply.

OrganiGram Holdings Inc (OGI.V) dropped 6.9 percent, Bedrocan Cannabis Corp (BED.V) fell 4.2 percent and Tweed Marijuana Inc (TWD.V) declined 2.8 percent, according to Reuters.

The Canadian government passed laws in April, which allowed licensed company to legally grow and ship medical marijuana, RT reported.

The potentially 1.3 billion Canadian-dollar industry is attracting the U.S. investors in peripheral aspects without being directly involved in the cultivation.

The laws in Canada allows licensed producers to sell marijuana with a doctor's prescription, and investors expect the industry to grow to $1.3 billion Canadian in a decade. Currently there are 13 licensed producers, RT reported.

Christian Groh, a co-founder of Seattle-based private equity firm Privateer Holdings, one of the largest players in the medical marijuana sector, told Reuters, "We really like the Canada model, which is really unlike any other in the world. What we're doing here does not violate local, state and federal law (in Canada)."

Timothy White, national risk specialist for Banker's Toolbox Inc., a firm that helps banks detect and report money laundering, said the move is illegal and could be charged as both drug trafficking and money laundering.

But no one has been prosecuted, largely in part to limited investigative sources, Payne said.