Terror threat

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.