U.S. security officials expressed concern Thursday about the possibility of Islamic State-inspired “lone wolf” terrorist attacks on American soil during the July 4 holiday weekend. Both the FBI and the Department of Homeland Security issued warnings after purported ISIS sympathizers carried out deadly attacks in France and Tunisia last month.

Authorities have not identified a specific threat or plot against the United States, the New York Times reports. However, the Islamic State’s recent calls for its followers around the world to carry out violence against the militant Islamist group’s enemies has raised concerns about potential “lone wolf” attacks, where individuals without direct ties to terrorist groups conduct violent acts on their behalf.

“We’ve been through other periods of high threat, but this time is different because of the proven reach of ISIS in Europe and the U.S.,” former National Counterterrorism Center director Matthew G. Olsen told the New York Times. “Even in the absence of a specific plot, there’s reason for alarm given the number of people who seem to have been motivated by ISIS’ rhetoric and direction to carry out attacks against innocent people.”

Last month, ISIS called on its followers around the world to use whatever means at their disposal to stage attacks during the Muslim holy month of Ramadan. “Make this month a month of victories for Muslims everywhere,” ISIS spokesman Sheikh Abu Muhammad al-Adnani said in an audio statement. “Make it a month of disasters, defeats and disgrace for the [infidels] everywhere.”

Federal officials cautioned local authorities to be on their guard this weekend amid the heightened threat, USA Today reported. The Department of Homeland Security took measures to boost security during Fourth of July weekend and told local police to be “vigilant” over the holiday.

U.S. Rep. Peter King (R-NY) told Fox News he had not seen federal officials this concerned about a potential terrorism threat since the September 11, 2011 attacks. “There’s always been something coming up, but nothing to this extent,” King said. “All the indicators are that ISIS does want to carry out an attack. We know they’ve had operatives here working in the country.”