WASHINGTON (AP) — Investigators believe a shooting that damaged power substations in North Carolina was a crime. What they don't have 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 plans 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. CISA, which falls under DHS, is tasked with helping reduce risk to physical as well as digital infrastructure across the country.