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, 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.

It came two months after two men opened fire outside a Prophet Mohammed cartoon contest in Texas before being killed by police, and during a year when several dozen people in the United States have been charged with supporting terrorism, with more than 10 arrested in the month before the July 4 holiday.

“This is the new normal,” said Will McCants, a terrorism expert at the Brookings Institution in Washington. If a terrorist group is looking to influence public opinion and generate fear, he said, “this kind of tactic has a lot going for it.”

One federal law enforcement official said investigators did not immediately find an extensive online presence involving the gunman, 24-year-old Muhammad Youssef Abdulazeez, or evidence that he was directly influenced or inspired by the Islamic State. The official was not authorized to discuss an ongoing investigation and spoke on condition of anonymity.

But the line between inspired and directed is blurred in an age of pervasive social media, where anyone with a computer or smartphone can be exposed to what FBI Director James Comey has called “poison” propaganda from terrorist organizations.

Law enforcement officials describe an ongoing challenge in distinguishing those who merely consume and share messages and those actually motivated to commit violence. Authorities say there’s no question that social media platforms, coupled with the small-scale plots being devised, have made terrorist ideology more accessible than a decade ago. It can be easy for those who read messages, but do not post their own thoughts, to avoid law enforcement scrutiny.

“They have now spent a year, maybe a little longer, investing in this strategy,” Comey told reporters last week, in a reference to IS. “And what you’re seeing now is proof that it works. Americans all over the place responding to this constant push and feed and buzz.”

The Kuwait-born gunman, who was killed by police, was not under investigation and was not on the radar of federal law enforcement before the shooting, officials have said. He had visited Jordan last year, a U.S. official said Friday, and investigators will review those overseas travels for potential worrisome contacts with militants.