U.S. officials are warning that public gatherings over the holidays are likely targets for terrorist attacks. The Department of Homeland Security released a statement Tuesday stressing that while authorities aren’t aware of any specific threats from terrorists, the holiday season provides “opportunities for violent extremists” to attack. The warning comes after an ISIS magazine called for the group’s followers to strike the Macy’s Thanksgiving Day parade in New York, saying it would be easy to use vehicles to launch an attack on outside gatherings. Authorities in New York have increased security for upcoming holiday events, but experts warn there is no real way to prevent terrorist attacks that target large, outdoor gatherings in open spaces. “It is very difficult to protect events like large gatherings such as parades from these types of attacks using vehicles, and we saw that last summer in Nice,” Matt Olsen, former director of the National Counterterrorism Center, was cited as saying by ABC News.