Holiday Season Could Be Target for Terrorists, DHS Says Warning bulletin follows call by ISIS to target parades, large gatherings.

 -- U.S. officials issued a somber warning today that the coming holiday season could mean "opportunities for violent extremists" to strike, especially as terrorist groups are squeezed abroad.

"Though we know of no intelligence that is both specific and credible at this time of a plot by terrorist organizations to attack the homeland, the reality is terrorist-inspired individuals have conducted, or attempted to conduct, attacks in the United States," reads a bulletin posted online today by the Department of Homeland Security.

The warning says terrorists could attempt to target "public events and places where people congregate."

The bulletin came days after an ISIS magazine called on the terrorist group's followers to use vehicles to attack popular outdoor attractions, like a New York parade.

"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," said Matt Olsen, a former director of the National Counterterrorism Center and a current ABC News consultant.

In July a man driving a large truck barreled into a packed crowd near the waterfront in Nice, France, killing more than 80 people before he was gunned down by police. ISIS later claimed in a statement that he was "a soldier of the Islamic State," though the group suggested that it did not have foreknowledge of the plot.

While a senior law enforcement official reiterated that there's no specific or credible threat at this time, the official said New York City's counterterrorism apparatus, from the police department to the FBI and other agencies, are ramping up special protocols used for major events.

John Miller, the NYPD's deputy commissioner for intelligence and counterterrorism, said Monday that the department has prepared for vehicle-borne attacks since the tragedy in France.

"The intelligence bureau identified 181 locations that rent trucks to the public in the metropolitan area. Of that, we went to 135 of those locations that rent vehicles under 26,000 pounds, where you wouldn't require the possession of a commercial driver's license to operate that. Our incident prevention unit provided them with very useful guidance on how to identify suspicious behavior and characteristics among people who are potential renters of those vehicles," he told reporters.

But Miller said he saw ISIS' threat more as "psychological warfare."

"What you see is the psychological warfare of printing materials that indicate to be afraid, be very afraid. We never cede to that," he said. "On the other hand, we have seen instances where [ISIS] has put out the call and people have risen to that call, which is why we have one of the most robust, complicated counterterrorism programs in the world and why we invest so many resources into it."

A counterterrorism official said that an ISIS-inspired attacker is of particular concern because the group may seek to "distract" the world from its battlefield loses in Iraq and Syria.

"As [ISIS] sheds its remaining safe havens, the group's pretensions of a caliphate look increasingly bankrupt, and desperation will likely fuel its pleas for action," the official said.