Vilifying and casting suspicion on American Muslims would threaten counterterrorism efforts and is un-American, the Homeland Security secretary said Monday at the nation's second-largest mosque, as Republican presidential candidate Donald Trump proposed banning all Muslims from entering the U.S.

Secretary Jeh Johnson, after asking Muslims to report anyone in their communities suspected of turning toward violence, declined to respond directly to Trump's proposal. Trump announced his idea during Johnson's meeting.

"I don't comment on what candidates for office say during this political season," Johnson said afterward. He would not otherwise answer questions about Trump's comments.

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