WASHINGTON (AP) — Investigators believe a shooting that damaged power substations in North Carolina was a crime. What they haven't named yet is a suspect or a motive.

Whatever the reason, the shooting serves as a reminder of why experts have stressed the need to secure the U.S. power grid. Authorities have warned that the nation's electricity infrastructure could be vulnerable targets for domestic terrorists.

Tens of thousands of people lost their electricity over the weekend after one or more people opened fire on two Duke Energy substations in Moore County, which is roughly 60 miles southwest of Raleigh. Nobody has been charged in the shooting as of Monday.

Here's a look at what is known about the shooting and why it could have implications across the U.S.

WHAT DO WE KNOW ABOUT THE SHOOTING?

The outages in North Carolina began shortly after 7 p.m. on Saturday when one or more people opened fire on two power substations in Moore County, the county's sheriff said. The outages left tens of thousands of people without electricity, and the equipment could take days to repair, according to Duke Energy.

Moore County Sheriff Ronnie Fields said at a Sunday news conference that authorities have not determined a motive. He said someone pulled up and "opened fire on the substation, the same thing with the other one." The sheriff said that it appeared gates were breached at both sites. The Pilot newspaper in Southern Pines, North Carolina reported that a wooden post holding up a gate had been snapped at one of the substations and that it was lying in an access road Sunday morning.

The sheriff noted that the FBI was working with state investigators to determine who was responsible. He also said "it was targeted."

"It wasn't random," Fields said.

Duke Energy spokesman Jeff Brooks said that the company has multiple layers of security at each of its facilities but declined to provide specifics. He said that the company has planning in place to recover from events like the shooting and that they are following those plans.

Department of Homeland Security spokesperson Ruth Clemens said the department's Cybersecurity and Infrastructure Security Agency has offered support to Duke Energy as it efforts the restoration of power.

TARGETS FOR EXTREMIST GROUPS

Federal authorities have warned that the power grid could be a prime target for extremist groups that embrace "accelerationism," a fringe philosophy that promotes mass violence to fuel society's collapse.

In January, a U.S. Department of Homeland Security report warned that domestic extremists have been developing "credible, specific plans" to attack electricity infrastructure since at least 2020. The DHS report warns that extremists "adhering to a range of ideologies will likely continue to plot and encourage physical attacks against electrical infrastructure."

The department wrote that attackers would be unlikely to produce widespread, multistate outages without inside help. But its report cautioned that an attack could still do damage and cause injuries.

Members of white supremacist and antigovernment groups have been linked to plots to attack the power grid. In February, three men pleaded guilty to conspiring to attack U.S. energy facilities. Authorities said they were driven by white supremacist ideologies to "sow mayhem and division among Americans."

OTHER ATTACKS

Fears of an attack on the nation's electricity infrastructure are nothing new. The Federal Energy Regulatory Commission ordered grid operators to increase security following a still-unsolved April 2013 sniper attack on a California electric substation.

The attack on the Pacific Gas & Electric Company's Metcalf Transmission Substation in an isolated area near San Jose, California, caused power outages and led to calls for millions of people to conserve energy.

The attack involved snipping fiber-optic phone lines and firing shots into the PG&E substation. The FBI said at the time that it found no evidence that it was an act of terrorism.

Former U.S. Sen. Mary Landrieu, who chaired the Senate Energy Committee in 2014, said at the time that it was fortunate the attack didn't cause a blackout in Silicon Valley, "the horrors of which could only be imagined."

In the wake of that attack, FERC and other agencies recommended utilities to take specific measures to protect vulnerable substations, like adding walls, sensors or cameras. Still, many remain exposed in rural areas of the U.S. And experts have warned for years that taking out a few substations could cause rolling blackouts in the U.S., leaving millions without power.

A Utah man was arrested in 2016 and later sentenced to federal prison time after he used a rifle to shoot the cooling fins of a substation, rupturing the radiator piping and causing the substation to overheat and fail. Court documents said the man had planned to attack other substations as part of an effort to take down power in a large chunk of the western United States.

WHAT'S THE CHALLENGE IN PROTECTING THE GRID?

The vastness of American electricity infrastructure makes it difficult to defend. Power plants and substations like those targeted in North Carolina are dispersed in every corner of the country and connected by transmission lines that transport electricity through farmland, forests and swamps.

"The grid is massive," said Erroll Southers, a former FBI official and professor of homeland security at the University of Southern California.

The targets also present an increasing challenge to secure because attackers don't always have to get as close as they did in North Carolina in order to do damage, Southers said. With the right rifle, skill and line of sight a sniper could take a shot from as far as 1,500 meters (about 4,900 feet) away.

Protecting substations against a long range rifle shot is "extremely challenging, if not impossible," he said.

Southers said all of these challenges mean that protecting the electricity infrastructure can come down to response and backup systems more than defense. "Those are the kinds of things that you put in place to protect, knowing that you may not be able to stop the rifle shot."

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Kunzelman reported from Silver Spring, Maryland.