U.S. intelligence officials have issued a warning about a possible attack by al Qaeda on Monday, the day before the presidential election, The New York Post reports.

The threat specifically concerns New York, Texas and Virginia, according to the report, although specific targets in those states were not identified.

Law enforcement authorities in New York City were said to be on high alert. In a radio interview, Mayor Bill de Blasio said: “We are still very much assessing the credibility. It is not at all clear how credible this is. We’re certainly in a vigilant position.”

De Blasio added that the NYPD is planning an “extraordinary” police presence for Election Day. Both Donald Trump and Hillary Clinton are having their election night parties in the city.

The New York City Marathon is scheduled for Sunday, and is another event that will see a heavy police presence.