SAN JOSE (AP) — Four U.S. senators concerned about the sabotage of Silicon Valley’s power grid and phone lines last April asked federal officials on Friday if mandatory security standards are needed.

The question was contained in a letter sent to federal energy regulators by Democratic Sens. Ron Wyden of Oregon, Harry Reid of Nevada, Dianne Feinstein of California and Al Franken of Minnesota.

They are asking if requiring power companies to comply with what are now voluntary security standards would improve grid safety.

The letter was sent just days after former Federal Energy Regulatory Commission Chairman Jon Wellinghoff said the attack on the grid was an act of terrorism. The incident is under investigation by the FBI, which says it has found no indication of terrorism.

The senators wrote that the sabotage was a “wake-up call to the risk of physical attacks on the grid.”

“This incident came uncomfortably close to causing a shutdown of a critical substation which could have resulted in a massive blackout in California and elsewhere in the West,” they wrote.

In December, Federal Energy Regulatory Commission chairman Cheryl LaFleur told Congress grid security is a top priority, and her agency cooperates with utilities to protect physical equipment.

Federal cybersecurity standards for protecting the grid are in place and mandated. However, rules for protecting physical sites such as transformers and substations are voluntary.

The senators said in their letter that they are concerned voluntary measures “may not be sufficient.”

Last April, a day after the Boston Marathon bombings, millions of people in Santa Clara County were asked to conserve energy after power lines were damaged.

At a news conference later that day, Sheriff Laurie Smith said someone had lifted heavy manhole covers at about 1:30 a.m. in two places on Monterey Highway south of San Jose, climbed under the road, and cut AT&T fiber optic cables, temporarily knocking out 911 service and phone service.

About 15 minutes later, someone fired a high-powered rifle into a nearby PG&E substation, damaging at least five transformers and causing an oil leak, she said.