Months since multiple shooting attacks on power substations in three states left thousands of people without electricity, investigators still don’t know who targeted the grid – or why.

Now, the FBI is offering a total of $100,000 in rewards for information about four shootings in Washington, Oregon and North Carolina. Steve Bernd, spokesperson for the FBI’s Seattle office, said it’s the first time his office has offered a reward related to a power substation attack.

“Interfering or tampering with our power grid can have deadly consequences,” said Richard Collodi, special agent in charge of the FBI’s Seattle field office. “An attack like this is not just an inconvenience for home and business owners. Think of medical facilities or vulnerable people who depend on electricity for their health.”

The call for information comes as concerns grow about increasing attacks on the power grid tied to rising domestic extremism in the U.S.

What happened in the power substation attacks?

Early on Nov. 22, about 5,200 people lost power after someone fired multiple rounds on the Barnes Lake Substation in Tumwater, Washington, according to the FBI. Puget Sound Energy found three 9 mm shell casings and fluid leaking at the scene.

Two days later and about 125 miles south, someone vandalized the Bonneville Power Administration Ostrander Substation in Oregon City, Oregon. Investigators found a hole cut in the perimeter fence and bullet holes in several reactors, the FBI said.

Officials have said it’s unclear what motivated the attacks or if they were related. “We announced the rewards and issued the posters jointly to raise visibility of the issue. We are looking at all possibilities at this point,” Bernd said.

The next month, someone fired multiple gunshots at two Duke Energy substations located 10 miles apart in Moore County, North Carolina. The Dec. 3 attack caused a “massive” power outage affecting 45,000 customers, with some losing power for as long as five days, the FBI said.

And then on Jan. 17, someone fired on another substation about 40 miles north in Randolph County. Investigators do not believe the North Carolina shootings are connected, according to the Charlotte field office.

The FBI offices field offices in Portland, Seattle and Charlotte are offering up to $25,000 for information on the respective incidents. The Moore County Sheriff’s Office is offering an additional $75,000.