"The courts must give us back our protective rights. Have to be tough," his tweet continued.
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Trump's comment comes after police shot and killed five people wearing fake bomb belts who staged a deadly car attack in a seaside resort in Spain's Catalonia region Friday, just hours after a van plowed into pedestrians on a busy Barcelona promenade.
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Spanish authorities said the back-to-back vehicle attacks - as well as an explosion earlier this week in a house elsewhere in Catalonia - were related and the work of a large terrorist group. Three people were arrested, but a manhunt was underway for the driver of the van used in Thursday's Barcelona attack, which killed 14 people and injured 100 others. The Islamic State group quickly claimed responsibility.
RELATED: At least 14 dead, 100 injured after Barcelona attack, 5 terrorists killed in raid