The FBI is joining local and state authorities in investigating purported "intentional attacks" on two electrical substations in North Carolina's power grid that left approximately 45,000 people without power and heat over the weekend.The Moore County Sheriff's Office said in a Saturday Facebook post that authorities are investigating the incident as a criminal occurrence after evidence indicated that "intentional" vandalism was apparent at multiple sites, including two substations that were damaged by gunfire.Experts have long warned about the vulnerability of the nation's electricity infrastructure, saying the the US power grid could be a prime target for domestic attacks.CNN chief law enforcement and intelligence analyst John Miller said these types of attacks have been a national concern for years, given the decentralized nature of the American electrical grid.The Saturday attacks come just one week after the Department of Homeland Security renewed a January bulletin warning that the US electric power infrastructure is an "attractive target" for domestic extremists."