The percentage of Americans selling their food stamps back to stores for cash has increased by 30 percent over the past several years, according to a new Agriculture Department study.

The study on food stamps trafficking — which the agency said included “covert investigation” in stores — compared the periods of 2006 -2008 to 2009 -2011.

Despite the increase, trafficking has declined since the 1990s, when the rate was nearly 4 percent of food stamps, also known as Supplemental Nutrition Assistance Programs benefits.

The total amount of SNAP benefits is now at roughly $858 million, compared to $330 million annually in the 2006-2008 period.

The increase reflects the overall growth in SNAP participation and benefits, the agency said in the August 2013 report.