Homeland Security Secretary Jeh Johnson said his department will be issuing new guidance to retailers this week giving them pointers on how to spot potential terrorists among their customers by looking at what they’re buying.

While saying the government cannot prohibit sales of some everyday materials, Mr. Johnson said retailers should be trained to look for anyone who buys a lot from what he described as a “long list of materials that could be used as explosive precursors.”

He said it was an extension of the “If you see something, say something” campaign launched by his predecessor, former Secretary Janet Napolitano, which tries to enlist average Americans to be aware of their immediate environment.

“We can’t and we shouldn’t prohibit the sale of a pressure cooker. We can sensitize retail businesses to be on guard for suspicious behavior by those who buy this kind of stuff,” Mr. Johnson said during a question-and-answer session after a speech at the Council on Foreign Relations.

Other than pressure cookers, Mr. Johnson did not say what other products might appear on the guidance that will be sent to retailers.

Mr. Johnson said he is aware of the tenuous balance between security and freedom, and does not want to upset it with his moves.

He also said there is no current credible information indicating that Islamic State militants, who are on the advance in Iraq and Syria, are plotting immediate attacks in the U.S.

He said, though, that there is a potential long-term threat, particularly given the number of foreign fighters from countries that have visa waiver privileges from the U.S.

He said the government should be looking for patterns in someone’s travel or purchasing that raise questions — particularly if they are buying what he called “explosive precursors.”

The Boston Marathon bombers used pressure cookers to create crude improvised explosive devices. The prospect had been considered by Homeland Security officials as far back as a decade ago, when the department issued a bulletin warning of the home appliance’s potential use as a terrorist device.

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