Media in Catalonia reported on Thursday, and counterterrorism sources appeared to confirm it, that the CIA tipped off authorities in Spain in May about a planned summer terrorist attack on Barcelona's Ramblas.
As well as going to national security forces, the CIA note went to the Catalonian region's autonomous police force, know as the Mossos d'Esquadra, the report said.
The news directly contradicts the Mossos' denials after the Barcelona attack that it had had any special intelligence information about terrorist plans.
The Mossos have been widely praised for their swift reaction and handling of the emergency but have faced criticism over not realising the risk of the religious imam who was believed to be the ringleader of the terrorists.
At a press conference on Thursday, Catalonian security officials admitted they were warned but that nobody, including national police, had felt it was credible information.
The new revelation about the CIA briefing adds pressure to the force, which is run by the regional government, and questions the ability of national antiterrorism police to coordinate with regional forces.