Intelligence bulletins sent out ahead of the holiday warn of copycat attacks and homegrown extremists targeting July 4 celebrations in City and San Francisco.

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.

Special events with high attendance and media coverage remain attractive targets for lone offenders and violent extremists seeking to cause mass casualties or draw attention to their causes.