Photo taken on March 16, 2020 shows the White House Visitor Center in Washington D.C., the United States. Photo:Xinhua

Investigators probing vandalism at electricity substations in the western US state of Washington appealed for public help in tracking down those responsible for attacks that have fueled concerns of a right-wing extremist campaign.

Local police gave no information on who they suspected was behind the vandalism, which knocked out power on Christmas Day for some 14,000 in Tacoma, a port city area south of Seattle.

Tacoma Public Utilities, which owns two of the facilities targeted on Sunday, said in a statement that it was alerted by federal law enforcement in early December about threats to its grid.

The Pierce County Sheriff's office appealed Tuesday for those near damaged substations to look at "surveillance video at your home or business... and let us know if you find anything that could help our detectives identify the suspect[s] responsible."

On the day of the attacks, the office said it was investigating but had made no arrests and did not know if it was a coordinated attack.

"It could be any number of reasons at this point... We have to investigate and not just jump to conclusions," it said, adding that it was aware of similar incidents in Washington, Oregon and North Carolina.

The vandalism follows warnings by US officials that neo-Nazis who say they want to spark a race war are targeting electricity infrastructure.

Violent extremists "have developed credible, specific plans to attack electricity infrastructure since at least 2020, identifying the electric grid as a particularly attractive target given its interdependency with other infrastructure sectors," the Department of Homeland Security said in a January intelligence memo, according to US media.

In early December, 45,000 homes and businesses in Moore County, North Carolina were left without power after someone used a high-powered rifle to damage two electricity substations.

In February, three men with neo-Nazi ties pleaded guilty in Columbus, Ohio to plotting to use rifles and explosives to damage power infrastructure in various locations. They pursued "a disturbing plot, in furtherance of white supremacist ideology, to attack energy facilities in order to damage the economy and stoke division in our country," Assistant Attorney General Matthew Olsen said at the time. And in 2021, five men who allegedly belonged to white supremacist and neo-Nazi online discussion groups were charged in North Carolina with planning attacks on power infrastructure.

They planned the attack to create "general chaos" as part of their "goal of creating a white ethno-state," the indictment said.

The Washington Post said after the Moore County case that law enforcement was investigating eight incidents in four states.