An April attack on Silicon Valley’s phone lines and power grid was terrorism, according to former Federal Energy Regulatory Commission chairman Jon Wellinghoff said Wednesday.

The FBI has released multiple statements that is has found no indications to support this claim, but Wellinghoff says he reached his conclusion after consulting with Defense Department experts about the attack, which involved snipping AT&T fiber-optic lines to knock out phone and 911 service, and firing shots into a PG&E substation, causing outages, an Associated Press article reports.

“This is the most sophisticated and extensive attack that’s ever occurred on the grid to my knowledge,” said Wellinghoff in the AP article. The FBI reports that the investigation is ongoing and urged people not to jump to conclusions.

However, statements from other high-ranking officials underscore Wellinghoff’s concerns about the power grid’s protection. Former CIA director Jim Woolsey said that video from the incident shows a group of three or four men conducting the attack in a “disciplined military fashion,” adding “This wasn’t hooliganism. This was a systematic attempt to take down the electric grid.”