Mexican authorities have seized plastic bags containing 2,000 fentanyl pills that were apparently baked into traditional bread rolls.

The seizure announced Tuesday by Mexico's National Guard suggests drug cartels are trying to piggyback shipments in packages of traditional Mexican products.

A specially trained dog alerted guard members to a suspicious package at an express mail delivery company in Culiacan, the capital of the northern state of Sinaloa. The state is home to the drug cartel of the same name.

When officers opened the box, they found packages of corn chips, and four traditional bread rolls. When they cut into rolls, they found lots of about 500 pills apparently baked inside each.

Mexico is the main supplier of counterfeit pills laced with fentanyl that kill tens of thousands of Americans each year.

The head of the Drug Enforcement Administration told CBS News that the

Sinaloa Cartel and the Jalisco New Generation Cartel are primarily responsible for the influx of fentanyl in the U.S.

"Those cartels are acting with calculated, deliberate treachery to get fentanyl to the United States and to get people to buy it through fake pills, by hiding it in other drugs, any means that they can take in order to drive addiction and to make money," DEA Administrator Anne Milgram told "CBS Mornings."

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