CHICAGO (CBS) — Customs agents seized 100 pounds of an illegal stimulant headed for Chicago, when the package was intercepted at Philadelphia International Airport, because the box felt too heavy to be what the label suggested it was.

U.S. Customs and Border Protection spokesman Steve Sapp said a phony label identified the package as topographical drawings. He said the shippers were creative in making a fake label, just not creative enough.

“The shipper identified the manifest as topographical drawings – you know, paper products – but when you pick up the box, and you realize right away that it’s a little heavier than just paper should be, you sort of get suspicious,” he said.

An X-ray scan of the box revealed it was filled with 100 pounds of the leafy green plant known as khat, grown in Africa and the Arabian Peninsula. Though popular as a stimulant in countries where it’s grown, khat is not a common street drug in the U.S.

“Communities that do have a larger population of folks that do come from Africa, that do come from Arabia, places like that, Southwest Asia, they’ll tend to be more users of that product,” Sapp said.

He also said khat has a very short shelf life.

“Within 48 hours, it starts to break down, and after a couple extra days, it becomes nothing more than mush,” Sapp said.

The khat would have had a $30,000 street value if it had made it through customs.

Khat is legal in other countries, like France, where the intercepted shipment originated, but in the U.S., it’s a Schedule I drug – considered by the federal government to be the most dangerous drugs, because the government recognizes no accepted medical use, and considers them to have a high potential for abuse.

Other Schedule I drugs include marijuana, heroin, ecstasy, LSD, and peyote.