Police in Barcelona have intensified security checks in some of the Spanish city’s most recognizable landmarks, following a security warning from the United States Department of State. The surprise warning came in the form of a post on the popular social networking site Twitter on Sunday, December 23.

In the tweet, the Department of State advised travelers to “exercise heightened caution around areas of vehicle movement, including buses”. It added that terrorists could “attack with little or no warning, targeting tourist locations, transportation hubs, and other public areas”. It is rare for the Department of State to issue warnings for specific locations, unless the US government is in possession of critical intelligence pointing to the possibility of a terrorist attack.

Hours after the Department of State’s warning, Miquel Buch, Minister of the Interior for Spain’s Catalonia region, told a radio station in Barcelona that local authorities were “engaged in assessing the warning” by the US authorities. Local media reported that increased police presence was visible around bus, minibus, train and metro stations throughout the Catalonian capital. Heavily armed police presence was also notable in Barcelona’s most popular tourist landmarks, including the Sagrada Familia Cathedral, the Gothic Quarter, and the mile-long Las Ramblas pedestrian Boulevard at the city’s center. There was no information about the precise nature of the US warning, but there were reports in Catalonian media on Tuesday that the alert notice involved the possibility of a vehicular attack by Islamists during the Christmas holiday season.

In August of 2017, Younes Abouyaaqoub, a 22-year-old Moroccan-born Islamist drove a van into large crowds of tourists at Las Ramblas, killing 14 and injuring nearly 150 people. Abouyaaqoub’s attack was followed by another assault by five men in Cambrils, a small seaside town south of Barcelona, who drove a car into a crowd of pedestrians, killing one and injuring six more. All six men were members of the Islamic State. They were shot and killed by police and security forces.