ISIS Threat at Home: FBI Warns US Military About Social Media Vulnerabilities

— -- The FBI on Sunday issued the strongest warning to date about possible attacks by the ISIS terrorist group against the U.S. military inside the homeland, officials tell ABC News.

In a joint intelligence bulletin issued overnight by the FBI with the Department of Homeland Security, officials strongly urged those who serve in uniform to scrub their social media accounts of anything that might bring unwanted attention from "violent extremists" or would help the extremists learn individual service members' identities.

"The FBI and DHS recommend that current and former members of the military review their online social media accounts for any information that might serve to attract the attention of ISIL [ISIS] and its supporters," the federal bulletin sent to law enforcement agencies said, advising that troops "routinely exercise operational security in their interactions online."

Officials said they fear copycat attacks based on what happened in Canada last month, when two uniformed Canadian soldiers were killed in two separate incidents by young men who claimed they were ISIS followers.

Some in U.S. special operations and other military branches have told ABC News they deactivated, scrubbed or locked Facebook and other personal social media accounts months ago at the urging of security officers as the U.S. began bombing ISIS in Iraq and Syria last August.

But the government on Sunday night indicated they had obtained fresh intelligence that ISIS wants to recruit or inspire sympathizers inside America to hurt military members where they live.

"The FBI recently received reporting indicating individuals overseas are spotting and assessing like-minded individuals who are willing and capable of conducting attacks against current and former U.S.-based members of the United States military," the bulletin said.

Attacks such as those in Canada -- which apparently were carried out without direct contact between ISIS and the perpetrators -- may "embolden" and "motivate" those who support ISIS, the FBI and DHS said.

Pentagon Press Secretary Rear Adm. John Kirby told ABC News the military isn’t saying service members should stop using social media altogether, but just to be careful when they do.

“We certainly do know that they [ISIS] monitor social media,” Kirby said. “I think there’s a general understanding here that his is an environment that this particular enemy has proven adept at using and we don’t want to give them any more advantage than they’re already trying to gain.”

Marie Jones, an active duty Army specialist, said she didn’t necessarily pay much attention to the warnings about social media, partly because she already makes an effort to keep military-oriented information off her pages.

“You just never know who’s out there watching or hacking,” Jones told ABC News.

The day before the U.S. launched its biggest air blitz against the terrorist group in Iraq and Syria in late September, ISIS spokesman Abu Mohammad al-Adnani called upon Muslims in the U.S. and Europe to attack members of the military.

"Do not ask for anyone's advice and do not seek anyone's verdict. Kill the disbeliever whether he is civilian or military, for they have the same ruling. Both of them are disbelievers. Both of them are considered to be waging war," Adnani said in an audio speech posted online on Sept. 21.

ABC News' Megan Chuchmach and Lee Ferran contributed to this report.