(NewsNation) — Officials are raising more warnings about infrastructure following attacks on at least four power stations across Washington State that left thousands without power on Christmas.

At 7 a.m. Monday, more than 105,000 Puget Sound Energy customers were without electricity in the Pierce County region, according to KOMO-TV, after a fourth electrical substation was vandalized late Sunday.

Earlier in the day, Tacoma Public Utilities had reported vandalism at one substation around 5:30 a.m., then later said that another substation was vandalized. Then, just before noon Sunday, Puget Sound Energy reported vandalism that caused a power outage around 2:30 a.m. at one of its own substations.

During all four incidents, officials said, someone forced their way into the fenced area surrounding the stations and damaged equipment.

As of Tuesday morning, most of the greater Tacoma area customers affected by the incident had their electricity restored, but authorities are still searching for the suspects who broke in.

“This person or people are still out there continuing to do this,” Sgt. Darren Moss of the Pierce County, Washington Sheriff’s Department, said. “You’re already affecting people’s lives, and to pick Christmas as a day to do it — what’s the message? What’s the reasoning for it?”

A powerful winter storm over the weekend had already left more than 500,000 homes and businesses in the dark nationwide. People in South Carolina watched their transformers blow as powerful winds moved across the area.

“First I woke up to like a big loud boom, I was sound asleep,” Judy Fowler, a resident of Spartanburg, South Carolina, said. “I I sleep near the window, so I looked out and I just saw these big giant balls of fire coming from the lines up there.”

Earlier this month in North Carolina, two substations damaged by gunfire left tens of thousands without power. The Department of Homeland Security issued a warning that infrastructure could be the target of domestic extremists.

In recent months, the United States has seen a wave of physical attacks and intrusions at power plants in Washington, Oregon, North Carolina, Florida and Hawaii. No one has claimed responsibility, but federal law enforcement has cited suspected white supremacists sharing information on specific substation locations.

Federal officials have already warned that the U.S. power grid needs better security to prevent domestic terrorism.

Tommy Waller, executive vice president at the Center for Security Policy, said when there are multiple substations damaged in the same area, it looks a lot like foul play and sabotage.

“It’s important that these events are investigated as though they’re meant by an adversary, whether foreign domestic, to turn off the lights, because turning off the lights will harm Americans extraordinarily,” Waller said.