At least three times in the last five months, U.S. military members have been urged to limit their social media activity in response to worries that ISIS-linked terrorists could track them down, in the U.S. or abroad.
The latest warning came this week, when a group calling itself the Islamic State Hacking Division posted personal information of about 100 service members, which defense officials said had been collected from social media sites.
The admonitions reflect growing concerns about "lone wolf" ISIS sympathizers who could try to carry out an attack in the U.S.
They also illustrate the power of social media in the hands of people who may identify with terrorist causes but are armed only with a laptop.
Pentagon Urges Caution After ISIS Publishes 'Kill List'March 23, 201502:16
Social media has widened the gap between the perception of a threat and how real that threat is, said Hugh Thompson, chief technical officer of the cybersecurity firm Blue Coat. "When that gap exists, it wreaks all kinds of havoc on the psyche," he said.
The warnings date back at least to September, when the Army Threat Integration Center published a report noting that ISIS supporters were using Twitter and other social media to threaten civilians and the military. A second came in December, when the FBI and the Justice Department urged military members to clean their social media accounts of anything that might attract the attention of violent extremists.
In the latest case, which became public last weekend, an online post threatened violence against specific service members, current and former, and included their names and addresses. Defense officials looked into it and determined that much of the information was already publicly available. In response, they advised the service members to stay off social media or limit their use of it.
The Defense Department has already gone to considerable lengths to educate service members on the risks of using social media, from posting geotagged photographs to writing about one's whereabouts. Its recommendations — deactivate location data, don't talk about coming travel, don't accept friend requests from anyone you don't trust — are similar to those that security experts advise for just about everyone, but with the stakes potentially higher.
"Both my children are in the military, and I tell them to be careful about what they're putting out there," said Gary Davis, chief consumer security evangelist at Intel Security. "Mostly because there's a permanence to it and could lead to people who could cause you harm."
Throughout history, military members have been told to be careful about revealing personal information. But that information has never been so easy to obtain.
"These circumstances are new, because the warfare, the type of conflict we confront today makes no distinction between a front line and a home front or between soldiers and non-combatants," said Brian Jenkins, a senior adviser at RAND and a combat veteran of the Vietnam War.
In the rapidly evolving landscape, there are terrorists, or terrorist sympathizers, who can demonstrate that they're part of the fight by spreading fear and alarm online, Jenkins said. This appears to be the case with the publication of the military members' names.
"They want to demonstrate that they, too, are part of the fight," Jenkins said. "They're dreaming up things they can do that will be upsetting or demoralizing to their targets."
NBC News reached out to many of the service members whose names were on the so-called ISIS hit list, and none of them would speak publicly. One, however, called it "a propaganda ploy" that was being fed by the media. "If we keep talking about it," he said, "it will be a big deal."
Sean Federico O'Murchu of NBC News contributed to this report.