U.S. investigators are becoming overwhelmed trying to keep up with the social media barrage by U.S.-based supporters of the Islamic State -- with the latest information suggesting "US military bases, locations, and events could be targeted in the near-term." 

The warning comes in a new, six-page bulletin obtained exclusively by Fox News. It warns law enforcement and specifically military personnel to be vigilant during upcoming national holidays and military events due to the "heightened threat of attacks by the Islamic State of Iraq and the Levant (ISIL)." 

Sent one day before the Memorial Day holiday weekend, the joint bulletin -- from the Federal Bureau of Investigation, Department of Homeland Security and National Counterterrorism Center -- advised there is no "credible" threat information targeting events on U.S. federal holidays. But it said, "we are aware of recent information suggesting US military bases, locations, and events could be targeted in the near-term." 

While the FBI and other law enforcement agencies have given generic warnings in the past, this bulletin spelled out the heightened chatter and advised precautions that should be taken. The list of "observable behaviors" also points to so-called insider threats, and warns about individuals asking "unusual questions" about building maintenance or security procedures. 

Importantly, it speaks to the sheer volume of social media activity by pro-ISIS users, and the challenge that poses for analysts and investigators. 

"The large number of social media postings by US-based ISIL supporters is challenging for investigators in differentiating those supporters focused only on promoting pro-ISIL rhetoric, which may be protected speech, vice [versus] detecting those prepared to engage in violence on the group's behalf," the bulletin said. 

The bulletin warned the "reach and popularity" of social media platforms has made it easier for U.S.-based extremists to "identify and connect" with foreign terrorist organizations, which can "potentially direct" them. It said the FBI estimates there are "hundreds, possibly thousands" of people in the U.S. getting "recruitment overtures or directives to attack the United States," and ISIS is using social media in "unprecedented ways" to send messages advocating attacks in the U.S. 

The bulletin warned that "given the recent uptick in social media usage by violent extremists, we continue to advise current and former US government employees and military personnel to actively review their online social media accounts for any information that could attract the attention of violent extremists and routinely exercise operational security in their interactions online." 

While the bulletin advised there is no "credible" threat information, DHS Secretary Jeh Johnson said at a National Governors Association meeting in February that the "credible" threat qualification in such statements has become "less and less relevant" as officials do not necessarily know when the next attack is going to happen. 

Despite a brief scare Sunday where Capitol Police found and destroyed a pressure cooker in a car near the National Mall during the Memorial Day concert, nothing hazardous was found. The next upcoming federal holiday is Independence Day.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.