San Diego Gas & Electric on Friday restored power to all 123,000 customers hit by an overnight outage in San Diego and Orange counties when two transmission lines tripped offline simultaneously near the Marine Corps base.

The blackout began at just after 11:40 p.m. Thursday. Power was restored by 3 a.m. Friday to all but 267 customers who could turn their lights back on by 10 a.m. Friday.

San Diego Gas & Electric spokeswoman Amber Albrecht says officials were still looking into the failure's exact cause, but they determined Friday that it started when two transmission lines tripped offline near Camp Pendleton after insulators in the area failed.

She says insulators typically fail when there is a debris build up.