Homeland Security Secretary Jeh Johnson Wednesday activated the National Terrorism Advisory System for the first time, warning the public of "self-radicalized actors who could strike with little or no notice."
The bulletin, which marks the addition of a new level of public warning, will be in effect for the next six months, Johnson said.
The bulletin states that the department "is especially concerned that terrorist-inspired individuals and homegrown violent extremists may be encouraged or inspired to target public events or places."
Johnson said there was no specific information about a pending attack.
"We are entering a new phase of the current threat," Johnson said.