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

FILE - In this July 17, 2015 file photo, an FBI investigator investigates the scene of a shooting outside a military recruiting center in Chattanooga, Tenn. The deadly shootings of Marines at two Tennessee military sites illustrate the threat FBI officials in recent months have warned about: violence by a lone gunman with apparent terroristic aspirations against a vulnerable government target. (AP Photo/John Bazemore, File) (The Associated Press)

A photo of Skip Wells is posted next to a traditional fallen comrade table at a memorial outside the JROTC office at Sprayberry High School where Wells attended, Friday, July 17, 2015, in Marietta, Ga. Wells was one of four Marines killed in Thursday's shooting in Chattanooga. (AP Photo/David Goldman) (The Associated Press)

WASHINGTON – 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.