Vandalism at four power stations in the western U.S. state of Washington over the weekend added to concerns of a possible nationwide campaign by right-wing extremists to stir fears and spark civil conflict.

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

Tacoma Public Utilities, which owned 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 their grid.

The Pierce County Sheriff's office said Sunday it was investigating but had made no arrests and did not know if it was a coordinated attack.

They said in a statement that they were aware of similar incidents elsewhere in Washington, in Oregon, and in North Carolina.

"It could be any number of reasons at this point. ... We have to investigate and not just jump to conclusions," they said.

But it follows warnings by U.S. officials that neo-Nazis who say they want to spark a race war are targeting electricity stations.

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 U.S. media.

Attacks in other states

In early December, 45,000 homes and businesses in Moore County, North Carolina, were out of 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 stations 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," said Assistant Attorney General Matthew Olsen at the time.

And last year five men who allegedly belonged to white supremacist and neo-Nazi online discussion groups were charged in North Carolina with planning attacks on power stations.

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

Jon Wellinghoff, the former chairman of the Federal Energy Regulatory Commission, said on CNN in early December that the Moore County attack resembled one on an electricity network substation near San Jose, California, in 2013.

In that case, which has never been solved, one or more people fired close to 100 rounds at the station, damaging 17 high voltage transformers at a cost of $15 million.

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