(CNN) The FBI helped Moldovan authorities three times in the last five years to thwart potential smuggling of nuclear and radioactive material, a Moldovan interior minister said Wednesday.

The cases in the former Soviet republic involved sting operations, and no one from jihadi groups was involved, a U.S. law enforcement official familiar with the investigations said.

But officials were concerned that smugglers would try to sell to extremist groups such as ISIS, the U.S. source said, and the cases offer a glimpse at efforts to keep such materials -- which in some cases could be used for weapons -- from being sold on the black market.

U.S. Rep. Adam Schiff, a California Democrat and ranking member on the House Intelligence Committee, said officials have been long concerned about radioactive and nuclear material being smuggled out of the former Soviet Union.

"We know that these radical Islamic groups like al Qaeda and ISIS would love to get their hands on radioactive materials to fashion some sort of bomb," Schiff said.

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