An attack that plunged tens of thousands of people in North Carolina into darkness has raised new concerns over a mounting threat posed by radical homegrown extremists declaring open war on the United States power grid.

But experts and former officials note that, despite a stated priority in combatting this threat, obstacles exist for law enforcement agencies in pursuing such measures against individuals and groups on U.S. soil.

In the lead-up to the midterm elections last month, Newsweek documented the digital dissemination of a wide array of plots, manuals and manifestos by domestic extremists seeking to incite acts of sabotage against energy sites nationwide, as well as recent real-life examples of such attacks taking place in various parts of the country. ...