Two men have been arrested in the Christmas day attacks that damaged four electricity substations in Tacoma, Washington that sparked fears of an extremist campaign, the Justice Department announced.

Matthew Greenwood, 32, and Jeremy Crahan, 40, were arrested on December 31 after being identified as suspects based in part on cellphone records, the department announced late Tuesday.

They were arrested with two unregistered short-barreled firearms, one with a homemade silencer, authorities said.

They were charged with conspiracy to damage energy facilities and possession of unregistered firearms.

The December 25 attacks knocked out power for around 15,500 homes and businesses in the Tacoma area south of Seattle.

According to the charges filed in federal court in Tacoma, the men broke into four separate substations, as much as 10 miles (16 kilometers) apart, manipulating or attempting to manipulate breaker switches to shut them down.

They did not steal anything, but caused an estimated $3 million in damages by their actions, according to the charges.

The charges said that in a post-arrest statement Greenwood confessed to the break-ins and claimed that the two wanted to disrupt power to be able to rob a business.

After shutting down two substations, they broke into a business aiming to steal from a cash register, according to Greenwood.

The charges did not explain why the men had damaged four substations and only broke into one business, and also did not link the two suspects to any political groups or motives.

But the investigation was led by an agent of the FBI's Joint Terrorism Task Force who has expertise in domestic extremism, according to the charges.

The Christmas vandalism followed warnings by US officials that neo-Nazis seeking to spark domestic conflict were targeting electricity infrastructure.

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.

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.