Reports that Eastern European criminals have been caught in the past five years attempting to sell radioactive materials to terrorists underscores the constant concern such activity poses for the city, NYPD Commissioner William Bratton said Thursday.

"It just reinforces that this issue is not going away," Bratton said when asked about a series of undercover operations reported by The Associated Press that nabbed suspects in Moldova accused of trying to sell dangerous materials such as bomb-grade uranium and cesium to extremists.

"It will remain a priority as it was for previous administrations," Bratton told reporters after a swearing-in ceremony for new cops in Queens.

Since Sept. 11, 2001, law enforcement officials have been concerned about efforts by terrorists to get their hands on radioactive material for use in a dirty bomb which, while not causing many immediate casualties, could contaminate scores of city blocks and require billions of dollars to remedy.

Considered less likely but of more catastrophic consequence would be detonation of a crude nuclear device made from enriched uranium that could kill hundreds of thousands and cause major economic damage, officials said.

"This was the nightmare scenario [Commissioner Ray] Kelly was talking about when he promoted Securing the Cities," said Rep. Peter King, referring to a $118 million effort by the Department of Homeland Security to give radiation detectors to the NYPD, as well as police in Suffolk and Nassau counties. Bratton said beat cops, police boats, aircraft and specialized units use such detectors and that every ship coming into New York harbor is screened.

King said that construction of a nuclear device requires skills and funding not readily available to terrorists. But a radiological dirty bomb, made from medical device isotopes like cesium-137 and cobalt-60, could be easily put together with conventional explosives, King said.

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Stuart Cameron, chief of support services for the Suffolk Police Department and an expert on nuclear terrorism, said there is obvious concern about a group like ISIS getting radiological materials. But in some cases criminal traffickers falsely claim access to enriched uranium, he said.