Attacks perpetrated by lone actors are the biggest terrorism threat to July 4th festivities in New York City and elsewhere, federal authorities said in a threat assessment obtained by CNN.

The FBI and Department of Homeland Security have said lone wolf attacks pose the biggest threats to July 4 celebrations this year in City and San Francisco. Intelligence bulletins sent out ahead of the holiday warn of copycat attacks and homegrown extremists,

As communities across the United States prepare to celebrate Independence Day, the FBI and Department of Homeland Security (DHS) have a warning. They are notifying local authorities about potential threats,