Three electricity substations in the Tacoma, Washington, area were attacked Sunday, leaving an as many as 14,000 customers affected, authorities said.
The agency estimated the number of homes and businesses affected by the attacks was 14,000 at one point Sunday.
In a statement, TPU said, "Two of our substations were deliberately targeted by physical attacks."
On Dec. 3, vandals attacked two Duke Energy substations in Moore County, North Carolina, leaving 45,000 customers in the dark for more than three days straight, officials said.
Investigators probing the North Carolina attacks were looking at online conspiracy theories to determine whether any played a role, two senior law enforcement officials briefed on the matter told NBC News earlier this month.