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U.S. intelligence officials have determined that Islamic State terrorists have tried to use the flow of hundreds of thousands of refugees from Syria to infiltrate into the United States, a leading House Republican said Monday.

House Homeland Security Committee Chairman Michael McCaul made the revelation, which he said came from a briefing this week, in a speech in which he called for immediate changes in the U.S. effort against the extremist group before it succeeds in launching another deadly attack on the homeland.

"I can reveal today that the United States government has information to indicate that individualts tied to terrorist groups in Syria have already attempted to gain access to our country through the U.S. refugee program," he said. "That was very courageous for them to come forward with this to tell me about this personally given the political debate on the Hill with the Syrian refugee bill."

McCaul also said President Obama has "doubled-down on a strategy of hesitancy and half-measures" against the group.

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