PHILADELPHIA (CBS/CNN) — A travel alert is now in effect for American citizens traveling to Europe.

The State Department says there’s a threat of terrorist attacks.

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Officials point to recent attacks in France, Russia, Sweden and the United Kingdom in issuing the warning.

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“While local governments continue counterterrorism operations, the Department nevertheless remains concerned about the potential for future terrorist attacks,” the alert said. “U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning.”

They also say terrorists tend to focus on popular tourist attractions.

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A U.S. official told CNN in February that the terror threat in Europe was as high as it’s ever been.

Recent estimates from Europe and U.S. say as many as 1,900 foreign fighters have returned to Europe after stints in Iraq and Syria, the official said. Some remain committed to terror groups like ISIS, but others returned disenchanted.

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The travel alert runs until Sept. 1.

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