The same PG&E electricity substation hit by gunfire in 2013 has endured a fresh security breach, despite a wide-ranging upgrade of the facility’s security systems, the utility reported Wednesday. Unknown individuals stole construction materials from the Metcalf substation in San Jose early Wednesday morning, PG&E said. The Santa Clara County Sheriff’s Office and the utility are investigating the incident.

"The preliminary review suggests human error as the apparent cause of the security breach," PG&E said in a prepared release.

According to PG&E, a breach of the fence at the facility triggered alarms at the substation, but those alarms were not detected by the utility’s onsite security staff for reasons the utility did not explain Wednesday.

A source with knowledge about the incident said that the security staff is supposed to make a sweep every 20 minutes of the perimeter, but the theft took at least 45 minutes to complete.

San Francisco-based PG&E had completed an array of upgrades at the Metcalf station prior to the theft of the equipment, said Nicole Liebelt, a spokeswoman for the utility.

The sniper attack in April 2013 — which remains unsolved but was described by at least former high-level federal official as a possible act of terrorism – prompted PG&E to undertake a sweeping upgrade of security at an unspecified number of electricity substations in California. PG&E is spending $100 million on the improvements.