The Islamic State is using social media to identify Americans overseas who can be targeted outside of a secure location, experts warned a Senate panel Tuesday.

"One of the emerging tactics that ISIS in particular is trying to use is stalking and killing its foes, especially those who are affiliated with governments," Daveed Gartenstein-Ross, a senior fellow at the Foundation for Defense of Democracies, told Sen. Joni Ernst, R-Iowa, during a Senate Homeland Security and Government Affairs Committee hearing. "Basically, taking them out of the government sphere, making them individuals who can be tracked."

Committee Chairman Ron Johnson, R-Wis., convened the hearing to discuss the threat of terrorism following the Brussels bombings last month. The panelists agreed that government personnel in Brussels and at various NATO facilities in Europe are at risk, but also noted the difficulty of providing adequate protection outside of government facilities.

"What some of the followers and adherents of ISIS have tried to do is expose military personnel and their family members with personal data, addresses, etc.," said Juan Zarate, a former deputy national security adviser under President George W. Bush. "So there is a vey real effort underway to at least threaten, if not put at risk, family members and personnel outside the bounds of classic security."

Social media has complicated that task, they added. "A lot of the information ISIS got when it put out addresses of service members who were on their kill lists was just easily gleaned, not from hacking, but from going to people's social media accounts and finding out this information about them," Gartenstein-Ross said.

"But ultimately, I think this is a very high-level concern that fits both with what the organization has done and also the direction that it's moving in with its evolving tactics."

Johnson invited Department of Homeland Security officials to testify at the hearing, but they declined, even though Homeland Security Secretary Jeh Johnson was at the Capitol for a separate event pertaining to funding for his agency.

"I think probably a pretty good way to try to secure those resources would be coming before a committee like this to lay out the reality of what the problem is," Ron Johnson said.