Radioactive metal has been found in the luggage of a passenger bound for Iran, Russian customs agents said.
Agents seized 18 pieces of metal at Moscow's Sheremetyevo airport after a radiation alert went off.
The gauges showed that normal radiation levels were exceeded by 20 times.
Prosecutors have launched a probe into the incident.
The pieces reportedly contained Sodium-22, a radioactive isotope of sodium that could only be produced at a nuclear reactor.
Sodium-22 is a positron-emitting isotope with a long half-life.
Spokeswoman Kseniya Grebenkina said the luggage had been seized some time ago, but could not specify when. The Iranian has not been detained.
Sodium-22 has medical uses, including in nuclear medicine imaging.
A spokesman for the Rosatom nuclear agency said the pieces are highly unlikely to have come from Rosatom and said the isotope is produced by particle accelerators, not by nuclear reactors. In Russia, universities, research institutes and big medical centres can have the technology to produce it, he said.
"There is an extremely slim chance that it could have come from Rosatom," he said.