WASHINGTON (AP) — Investigators believe a shooting that damaged power substations in North Carolina was a crime. Tens of thousands of people lost their electricity over the weekend after one or more people opened fire on two Duke Energy substations. Nobody has been charged in the shooting as of Monday, and authorities don’t yet know the 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. An expert says that the sheer size of the electric grid can make it difficult to defend.