Contractor accused of tampering with power grid's computer system

Lonnie Charles Denison, 32, was employed by an unnamed company that contracted with the Independent System Operator, said a spokeswoman for the agency. He was arrested around 1:30 p.m. by the FBI and faces federal charges of destruction of an energy facility, officials said.

On April 15 around midnight, authorities began investigating why the internal power supply had been cut off to several computer systems that handle ISO market data. The ISO oversees electricity purchases and distribution in California.

The computers were fully restored by 6 a.m., officials said, and built-in redundancies prevented the tampering from affecting the power grid.

On April 16 around 12 noon, the ISO received a "credible threat" to the facility through an e-mail "from the same suspect," officials said. Five hundred employees in all three ISO buildings at the Folsom campus were evacuated, according to the agency.

Authorities searched the building but found no potential problems and allowed employees back in at about 5 p.m. During that time, grid operations were transferred to another facility, and service was not affected.

A spokeswoman said the ISO is conducting an internal investigation to "review security policies and procedures."