SACRAMENTO – Federal agents on Wednesday arrested a contract employee they say tampered with computers at the headquarters of the agency that controls California’s electricity transmission. Lonnie Charles Denison, 32, of Sacramento faces a felony charge of destruction of an energy facility. The crime carries a maximum sentence of five years in prison and a $5,000 fine. Denison is being held in Sacramento County Jail and is scheduled to be arraigned today in federal court in Sacramento. The tampering, coupled with a bomb threat at the agency’s Folsom headquarters Monday, had drawn the attention of the FBI’s Joint Terrorism Task Force because of the potential impact on the state’s power grid. The California Independent System Operator manages 25,000 miles of electrical transmission lines and operates wholesale power markets. Stephanie McCorkle, spokeswoman for the agency, said employees at the Independent System Operator were relieved to learn of the arrest. McCorkle said neither the tampering nor the bomb threat disrupted the agency’s operations. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! “Employees are obviously feeling more comfortable,” she said. The tampering began at 11:30 p.m. Sunday and continued into the early morning hours Monday, according to the FBI’s arrest affidavit. The affidavit said Denison, hired to operate computers, used a security key card to gain access to a data center at the facility. Once inside, Denison used a hammer to break a glass plate covering an emergency power-off button. By pressing the button, he cut the facility’s internal power supply to the majority of the data center, preventing the agency from controlling computers used to buy and sell energy in real time. Denison was working for Science Applications International Corp., an information-technology firm that had performed contract work for the agency for more than a year, McCorkle said. His security clearance had been revoked last week after a dispute with his employer, the affidavit said. He had twice tried to access the agency’s computer system remotely Sunday but was denied.