A 33-year-old man from Sierre Leone was arrested at John F. Kennedy International Airport on Wednesday after agents found uranium in the sole of his shoes.

Federal prosecutors say Patrick Campbell was charged after a flight from Paris. The yellowcake uranium did not pose immediate harm, NBC New York reports.

"Campbell assisted the agents in removing the Uranium from beneath the inside soles of his shoes, and plastic bags containing Uranium were recovered from two of Campbell's shoes," according to a criminal complaint.

Over the past several months, Campbell had unknowingly been communicating with an undercover U.S. Homeland Security agent, saying he had connections to a uranium mine in Sierra Leone and agreed to sell 1,000 tons of the uranium powder to Iran, the New York Daily News reports. He had agreed to travel to Miami to have his sample analyzed for purity, Reuters reports.

Campbell made a court appearance on Thursday and faces up to 20 years in prison and a $1 million fine if found guilty, Reuters reports.

The federal investigation, which began in May 2012, was prompted by an ad on alibaba.com seeking the purchase of yellowcake uranium. Campbell responded to the ad and began communicating with federal agents over email, Skype and telephone calls, thesmokinggun.com reports.

He assured agents that his firm had experience with selling uranium to countries like China and Ecuador. He added that masking the radioactive element “will not be a problem” since he “will mix the Uranium with Chromite Ore to disguise it.”

Yellowcake uranium, also known as U3O8, is a raw form of uranium that contains a small amount of the radioactive element. It’s the first step toward enriched uranium, which can be used to make nuclear fuel and nuclear weapons.

When Campbell was arrested, prosecutors said they found raw uranium in two pairs of shoes and a USB drive containing a contract for the sale and supply of uranium to Iran.