A security threat forced the evacuation Monday of the Folsom offices of the California Independent System Operator facility, which controls the state's power supply.

ISO, along with state and federal authorities, have launched an investigation into the interruption of power to certain computer systems late Sunday.

At no time was there an impact to the power grid, officials aid. Power was restored by about 6 a.m.

About 11 hours after the power went out, the ISO received a threatening e-mail.

Authorities were very tight-lipped about the contents included in the e-mail.

They would not say who it came from, where it came from or why they felt compelled to evacuate all 450 people at the ISO offices. Grid operations were transferred to another facility, officials said.

Authorities did not say if there was any connection between the outage and the e-mail.