Military installations from Cape Cod to El Segundo, Calif., have stepped up security amid FBI warnings that the Islamic State is trying to incite its followers in the U.S. to launch domestic terrorist attacks.

And experts say the next step to that threat is when American ISIS sympathizers they’re tracking online suddenly “go dark.”

“I think the fact that these guys are going dark now is a recognition on their part that if they stay out in the open that the FBI is going to catch them,” said Steven P. Bucci, director of the Allison Center for Foreign Policy Studies at The Heritage Foundation.

“So in addition to polishing their social media skills they’re also getting better at trade craft as far as protecting their folks so that they can potentially do something here before the FBI can get a hold of them,” Bucci said.

The security level at Department of Defense bases has been raised from Alpha — the lowest — to Bravo for the first time since Sept. 11, 2011, the 10-year anniversary of the 9/11 attacks, according to Preston Schlachter, spokesman for the U.S. Northern Command, or Northcom. The order was signed by Adm. William Gortney, head of Northcom.

The call for beefed up security came after FBI Director James Comey warned of “hundreds, maybe thousands” of Americans following ISIS accounts online.

Comey said the Islamic State is leveraging social media in unprecedented ways through Twitter and other platforms, directing messages to the smartphones of “disturbed people” who could be pushed to launch assaults on U.S. targets.

“The siren song sits in the pockets, on the mobile phones, of the people who are followers on Twitter,” Comey said. “It’s almost as if there’s a devil sitting on the shoulder, saying ‘Kill! Kill! Kill! Kill!’ all day long.”

Bucci noted that ISIS is “the best at social media that we’ve seen. Their followers are looking at it and saying, ‘OK if we can’t get in the main show in Iraq or Syria, I’ll do something here.’ ”

ISIS has already taken credit for the shootout in Texas outside a contest to draw cartoons of the Muslim Prophet Muhammad that left both gunmen dead and a security guard wounded.

“It’s clear that, unlike al-Qaeda, ISIS has done a very successful job of doing outreach through social media and the Internet,” said Bradley Schreiber, a former senior adviser at the U.S. Department of Homeland Security. “They’ve been able to attract foreign fighters at a level never seen before. Al-Qaeda’s efforts were more localized. ISIS has international outreach through Western Europe and now North America.”

Added Edith Flynn, terrorism expert at Northeastern University: “I think what ISIS is doing is trying to expand its footprint across the United States and I think there is a growing problem with self-radicalization. And I think it is going to continue.”

Military installations in the Bay State that are on higher alert include Hanscom and Westover Air Force bases; Otis National Guard Base on Cape Cod; Army bases Fort Devens and the Soldier Systems Center in Natick, and Sector Southeastern New England Woods Hole.

Schlacter of Northcom said “there is no specific threat tied” to the order, but security analysts believe there must have been some concrete intelligence that prompted officials to increase security.

“There are all types of threats, and the government tends to respond to those that are real and credible threats,” Schreiber said. “There’s a whole lot of intelligence out there that’s being collected and analyzed. If they’re going to go through the process of raising the threat level, you can infer that there’s probably something out there that they’re looking at.”