A South Natomas resident accused of shutting down computers that manage energy at the nonprofit that controls the flow of power over California's transmission grid pleaded guilty to a felony charge Friday in U.S. District Court in Sacramento, prosecutors said. Lonnie Charles Denison, 33, was a contract employee managing computer applications at the California Independent System Operator in Folsom, said Assistant U.S. Attorney Kyle Reardon in a news release. At 11:15 p.m. April 15, Denison turned off power to the ISO computers that communicate with the electricity market. prosecutors alleged.

Denison first tried to log into the ISO's computer system, but was unsuccessful because his computer privileges had been suspended, investigators said. Denison broke a glass cover that protected an emergency shut-off button, meant for use in case of fire or other calamity, according to the release.

Denison allegedly went home after shutting off the power to the computers, and told a friend the next morning that he had pressed the emergency button and had tried to "shut off the power grid," according to prosecutors.

The ISO manages high-voltage lines owned by Pacific Gas and Electric Co., Southern California Edison and San Diego Gas & Electric Co. The April 15 outage disrupted CalISO communications for about two hours, leaving the grid vulnerable to electricity shortages. The ISO used back-up systems to reconnect to the energy market.

Denison is scheduled to be sentenced by U.S. District Judge Garland E. Burrell Jr. at 9 a.m. on Feb. 29. The maximum penalty for attempting to damage an energy facility is five years imprisonment, a $250,000 fine and a three-year term of supervised release. Denison originally had also been charged with making a bomb threat.

The case was investigated by the Federal Bureau of Investigations's Joint Terrorism Task Force, composed of federal, state and local law enforcement agencies.