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Once again, there are flickering candles and flowers left on the streets of one of the world’s great cities to mourn terror attack victims. This time, blood has been shed in and near the breathtaking Spanish city of Barcelona, a global tourism magnet known for its striking modernist basilica and vibrant street scene.

At least 14 people were killed when ordinary vehicles became weapons on Thursday and early Friday and mowed down people in two locations. One assault took place on the crowded sidewalks of a popular promenade in the heart of Barcelona. The other happened in Cambrils, a smaller city on the Mediterranean coast about 60 miles away. Thoughts and prayers from around the world are still a necessary balm for grieving families and a wounded nation. May we never grow so accustomed to ongoing terror attacks that we halfheartedly mourn innocent lives lost and fail to pause to denounce their murderers.

These words, however, must be accompanied by deeper determination to thwart extremists of all ideologies. Initial reports indicate that the Barcelona attacks were planned by the Islamic State of Iraq and Syria and carried out by multiple people. This distinguishes it from a so-called “lone wolf” attack by someone with more tenuous ties to terrorism. The Barcelona attack’s scope and coordination suggests that ISIS still has enough organizational resources to execute its hellish ambitions.

International cooperation is critical to track down the perpetrators and prevent another tragedy. Other nations have long been partners in the fight against terror. President Donald Trump should strongly signal the world that he values this collaboration and will seek to wield the world’s resources to pursue justice. While a tweet would be helpful, this message would resonate far more strongly if Trump employed a more traditional presidential communications strategy.

The terror attacks in Spain follow last weekend’s violence in Charlottesville, Va., spurred by right-wing extremists. The run of such tragic events may tempt some to tune out the news. Don’t succumb to this. The Charlottesville attack drove home that terror flourishes within our borders as well as in faraway deserts. Every American shares the responsibility to work peacefully to stop it.