Military site shootings crystallize FBI terrorism concerns

WASHINGTON (AP) — The deadly shootings at military sites in Tennessee illustrate the threat that FBI officials have warned about: violence directed against a vulnerable government target by a lone gunman with apparent terrorist aspirations.

The FBI has not detailed a motive, but Thursday's attacks that killed four Marines and one sailor are under investigation as a potential act of terrorism, with authorities combing through the gunman's past to look for travel, contacts and online writings.

The rampage unfolded as the federal government has raised alarms about the online spread of terrorist propaganda, including repeated exhortations by groups such as the Islamic State for sympathizers to target police officers and military installations.

Law enforcement officials describe an ongoing challenge in distinguishing those who merely consume and share messages and those actually motivated to commit violence.

Federal officials have warned over the last year that today's terrorist sympathizers can be difficult to track and predict, in part because their plans are often spontaneous, poorly formed and lack the sophisticated execution that has defined al-Qaida and its operatives.