Updated at 11:50 a.m. ET
NBC News reported that U.S. officials believe the type of radioactive material seized was typically used for medical diagnostics, pointing away from Iran's nuclear program.
It quoted an un-named U.S. intelligence official saying: "We don't know whether the receipient was a government agency. It's unclear why they would want to smuggle it in - we are still investigating - but it does not appear that this involves Iran's nuclear program."
It said the official was nonetheless "deeply concerned" by the incident and commended the Russian security services for seizing it.
Updated at 7:43 a.m. ET
MOSCOW - Radioactive material has been seized from the luggage of a passenger bound for Tehran, Russia's customs agency said Friday.
Authorities said agents found 18 pieces of metal at Moscow's Sheremetyevo international airport.
In a statement, Federal Customs Service said that tests showed the material was a radioactive isotope which could be obtained only "as a result of a nuclear reactor's operations."
Gauges showed that normal radiation levels were exceeded by 20 times, officials said.
Citing the customs service's statement, NBC News reported that the pieces of metal were individually packaged in steel canisters.
The Interfax news agency reported that the pieces of metal contained Sodium-22, a positron-emitting isotope with a remarkably long half-life. It can also be used in medical equipment, Reuters reported.
The customs agency said prosecutors have launched a probe into the incident.
Tension is escalating between Western powers and Iran after a U.N. nuclear watchdog report last month that said Tehran appeared to have worked on designing a nuclear weapon, and that secret research to that end may be continuing.
The United States and its European allies have seized on the unprecedented document by the U.N. International Atomic Energy Agency (IAEA) to crank up the sanctions pressure on Iran, one of the world's largest oil producers.
Iran says its nuclear program is for peaceful purposes.
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Reuters, The Associated Press, msnbc.com staff and NBC News contributed to this report.