Mossos d'Esquadra officers believe that a bus, minibus, or similar vehicle could be used during the attack. Police officers have identified a man who could possibly be a bus driver, according to Antena3.
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Regional police forces have started conducting checks on buses and areas that could be possible terrorist targets, especially in downtown areas.
Earlier, the US State Department has issued a warning to tourists planning to visit Barcelona, warning about the increased security threat.
Last year, Barcelona was the scene of several van-ramming attacks, which killed a total of 16 people and injured more than 150.