The daily El Periodico de Cataluna reported that the US National Counterterrorism Center (NCTC) alerted Spanish intelligence officers to the threat weeks before a man ploughed his vehicle into crowds of tourists along Barcelona's Las Ramblas boulevard on August 17, killing 14 people.
"Unsubstantiated information of unknown veracity from late May 2017 indicted that the Islamic State of Irak and ash-Sham (IS) was planning to conducted unspecified terrorist attacks during the summer against crowded tourist sites in Barcelona, Spain, specifically (Las Ramblas) street," according to an NCTC briefing note published by the paper.
The interior ministry did not respond to a request for comment.
But the Madrid-based Cadena SER radio station quoted "anti-terror sources" suggesting that the document published by El Periodico de Cataluna was authentic.
Spain is still recovering from the twin August vehicle attacks that left a total of 16 people dead and more than 120 wounded in Barcelona and the seaside resort of Cambrils further south.
IS claimed both attacks.