SAN JOSE — PG&E on Thursday offered a $250,000 reward for information leading to the arrest and prosecution of anyone involved in the gunshot attack last year on the utility’s Metcalf substation in south San Jose, and said it is spending $100 million to improve security at its substations.

Disclosure that PG&E is increasing security comes amid questions about the vulnerability of the electricity systems operated by utilities nationwide.

The FBI on Thursday asked for public assistance in its investigation into the PG&E substation shooting in April of last year, which caused $15.4 million in damage and cut some AT&T fiber optic cables.

“We’ve been working with the FBI and other law enforcement agencies and in our discussions with investigators, they believe a beneficial reward would be helpful in the investigation,” said Gregg Lemler, vice president of electric transmission operations with PG&E. “We want to help however we can.”

Federal investigators urged anyone with information to contact their nearest FBI office or dial 911. Tips can also be submitted online at tips.fbi.gov.

Some experts believe that the attack suggests that electricity utilities are vulnerable to attacks from an array of culprits, including terrorists.

“The electricity grid is pretty vulnerable,” said Thomas Carlton, chief executive of Illinois-based Infrastructure Defense Technologies. “Electricity systems throughout the country are wide open to attacks.”

San Francisco-based PG&E is adding more security guards to provide 24-hour coverage at the Metcalf substation, trimming back undergrowth around substations to eliminate hiding places, adding fencing and shielding to obstruct views and protect substation components, installing enhanced cameras, and increasing lighting.

Yet that’s not enough, in Carlton’s view.

“PG&E has to look far beyond the tactics that were used at Metcalf last year,” Carlton said. “They have to look way ahead in terms of threats. They should consider some of the preconceived threats that are used in planning for protection of nuclear power facilities.”

Substations should be guarded against intentional crashes of large vehicles, all kinds of shoulder-fired weapons and any kind of a protection breach, Carlton said. Electricity substations should install layers of security that would pose complex countermeasures against even a sophisticated attack, he added.

“We just don’t know how PG&E is planning to deal with this long-term,” Carlton said.

In the near term, PG&E intends to spend $100 million over the next three years for multiple upgrades and improvements and additional security guards at its critical substations, Lemler said. PG&E wouldn’t identify which substations would receive the upgrades, other than to say the improvements are underway at power substations beyond the Metcalf power facility.

“Today’s economy and our quality of life depend on electricity,” said Geisha Williams, executive vice president of electric operations for PG&E. We are committed to taking aggressive action to ensure that our facilities are protected so we can continue to deliver safe, reliable and affordable service to our customers.”

Contact George Avalos at 408-859-5167. Follow at twitter.com/georgeavalos