Far-right groups are increasingly talking about attacking the US power grid to cause chaos and advance their cause, terrorism experts say.
The warnings come as the founder of a neo-Nazi group and a woman he met in prison are scheduled to appear at a plea hearing on Friday.
They are charged with plotting to attack power installations around Baltimore.
Brandon Russell, 27, and Sarah Clendaniel, 34, face up to 20 years in prison if convicted.
In messages revealed in court filings, Ms Clendaniel described their alleged plot as "legendary" and hoped it "would completely destroy the whole city".
The pair were arrested before the alleged attack was carried out. Prosecutors said thousands would have been left without power if it had gone ahead.
Attacks against infrastructure are a long-standing obsession of far-right and white nationalist groups, and they are increasingly being discussed in extremist spaces online.
Veryan Khan, president and CEO of the Terrorism Research & Analysis Consortium (Trac) says attacks are "not a matter of if, it's when".
Recent attacks
In addition to the alleged Baltimore plot, investigators are looking into several recent attacks on power installations, including incidents in North Carolina, Oregon and Washington state.
The North Carolina attack, in Moore County in early December, knocked out power to 35,000 people for several days. Nobody has been arrested and the investigation is ongoing.
Authorities say there's no known connection between the alleged Baltimore plot and the North Carolina attack, and the motive for most of the other attacks is unknown.
Mr Russell is the founder of Atomwaffen Division, a neo-Nazi group that has been linked to murders, bombings and plots in the United States and other countries. It is banned in the UK and Canada. He spent time in prison on explosive charges, and while locked up met Ms Clendaniel, who was convicted of armed robbery, according to an indictment.
Once they were released, the pair began allegedly planning to attack electricity substations in the Baltimore area, looking at openly available material about power stations and attempting to obtain a gun. But - prosecutors claim - a person who they thought was a potential co-conspirator was actually an FBI source.
The pair are scheduled to appear in federal court in Baltimore on Friday. Mr Russell's lawyer told the BBC he would plead not guilty. A lawyer for Ms Clendaniel did not respond to requests for comment.