When it comes to FBI threat information, there are two phrases that are used quite a bit: "out of an abundance of caution" and there is no "credible threat." In the post 9/11 world, information sharing between the FBI, Department of Homeland Security and state and local law enforcement agencies is the key.
Each year, the FBI and DHS send out hundreds of "Joint" law enforcement bulletins to numerous agencies nationwide. Often, these contain specific information that is threat related. Many of these bulletins detail tactics or techniques used by criminals and terrorists worldwide. For instance, a recent bulletin discussed a European terrorist who was hiding explosive blasting caps in his shoes. The FBI simply wanted local law enforcement to know that this was a tactic that was used in Europe and could be used here, even though there was no intelligence to suggest that it was being used by terrorists targeting the US.
Additionally, the federal government's intelligence community sends out numerous IIRs, Intelligence Information Reports, each year. The FBI alone sends out thousands of them to other federal agencies and those agencies, such as Secret Service and DHS send many in response. IIRs, like one recently made public detailing a threat against shopping malls in Chicago and Los Angeles, contain raw intelligence information. The information comes in and before it can be vetted or checked out, its put out, usually in a timely manner for the sake of sharing the threat. "When in doubt, put it out" is how these are described by FBI Special Agent Richard Kolko.
Besides the bulletins, the FBI also puts out hundreds of Intelligence Assessments every year. These include information on terrorists and regular crimes, such as gangs and organized crime.
And besides the paper sharing, the FBI and local law enforcement often sit at the same table, between the Joint Terrorism Task Forces and the Field Intelligence Groups at each FBI Field Office around the country. Some cities also have new "Fusion" centers where local law agencies of all stripes share office space with numerous federal agencies, so that information is shared as it comes iN.
The point of all the information sharing is common sense, the more people who know about the threat, the more agencies, officers and cops can help protect against it. And it's out of an "abundance of caution" that this information is shared, so that even if there is no "credible threat" against the homeland, at least the government at all levels knows what's out there. In this post-9/11 world, no federal agency wants to be accused of failing to share a piece of intelligence that could have prevented or thwarted a crime…. or worse, a terrorist attack. So it would be a mistake to assume that every time information is shared between law enforcement agencies, that a terrorist threat -- is imminent.