Tennessee shootings highlight law enforcement concerns over terrorism, attacks by lone gunmen

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.

Authorities are combing through the gunman's past to look for travel, contacts and online writings.

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