WASHINGTON—U.S. investigators are probing whether a suspected terror plot against Western European targets extends to the U.S., officials said and The Wall Street Journal reports.

Intelligence collected primarily points to potential threats targeting European countries, including the U.K., France and Germany, U.S. officials said. But after a spate of thwarted or botched plots against the U.S. this year, investigators are looking hard at any potential connection across the Atlantic.

"You have folks increasingly concerned about: Is it not just Europe that needs to be careful, but is there a threat here as well?" one U.S. counterterrorism official said.

The Central Intelligence Agency has stepped up drone strikes in Pakistan in an effort to help thwart the plot, which could still be evolving. The more than 20 strikes this month represent a monthly record, according to a tally by the New America Foundation.

Several U.S. officials said they haven't seen a terror threat as serious as the suspected European plot for many years. "This isn't just your typical Washington talk about how the threats have evolved. People are very concerned about what they're seeing," the counterterrorism official said.

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