WASHINGTON — The government plans to give U.S. retailers a list of materials that can be used in explosives and ask them to look out for suspicious behavior as a way to combat domestic terrorism, the homeland security secretary said on Wednesday.

On the eve of the 13th anniversary of the Sept. 11 attacks, Jeh Johnson said the so-called home-grown or “lone wolf” attack like last year’s Boston Marathon bombing was the hardest to detect and the one he worried about most.

He said his department would issue a public advisory to retailers this week, identifying the materials and suspicious behavior to look for in anyone buying large quantities of them.

Homemade bombs in pressure cooker pots were placed at the finish line of the famous race in Boston on April 15, 2013, killing three people and injuring 176 others in the worst attack on U.S. soil since Sept. 11, 2001.

“So we can’t and we shouldn’t prohibit the sale of a pressure cooker,” Johnson told the Council on Foreign Relations in New York. “We can sensitize retail businesses to be on guard for suspicious behavior by those who buy this kind of stuff.”

Johnson cited community outreach as another important element in countering violent extremism within the United States, including in immigrant communities whose youth might be drawn to joining Islamist fighters in Syria and elsewhere.

Johnson’s remarks came as President Obama prepared to set out a long-term strategy to defeat Islamic State, including air strikes against the group in Syria, in a speech to Americans on Wednesday evening.

While there is no credible threat from Islamic State on U.S. soil today, the group is an extremely dangerous mix of terrorist organization and insurgent army that has Americans and the United States in its crosshairs, Johnson said.

“Despite its slick public media and its self-proclamation to be the Islamic State of Iraq and the Levant, ISIL is neither ‘Islamic’ nor is it a ‘State,'” Johnson said, noting that most of the people the group kills are Muslims.

“ISIL is a stateless group of depraved criminals, rapists, kidnappers, killers and terrorists who control territory,” he said. “There is no religion, including Islam, and there is no God, including Allah, that would condone ISIL’s violent tactics.”