By . Lizzie Parry . Experts in the United States have warned an assault on a Californian power station in April could have been the 'dress rehearsal to a larger terrorist attack'. On April 16, last year, as many as two gunmen stormed the PG&E Metcalf substation in California after severing phone lines and firing several dozen rounds at transformers. At . least one person, maybe two, went down multiple manholes at the . facility in a San Jose suburb and cut fiber cables leading to the . substation. Attacked: The FBI is investigating an April attack on PG&E's Metcalf substation. Experts have warned the assault could have been a 'dress rehearsal for a larger attack' This knocked out 911 and landline service to the power station, as well as mobile phone service to the surrounding area. PG&E employees had no means to call for help when, at 1am, gunmen began their attack. More . than 100 rounds were fired from high-powered rifles at many . transformers – 10 were damaged in one area and three transformer banks . in another, a PG&E spokesperson said at the time. Cooling . oil leaking from at least one transformer bank caused transformers to . overheat and shut down. This led officials to warn locals to conserve . energy, but no major power outages occurred. Former chairman of the Federal Energy Regulatory Commission, Jon Wellinghoff, has today called the attack 'the most significant incident of domestic terrorism involving the grid that has ever occurred'. He told the Wall Street Journal he has become 'increasingly concerned' that a larger attack could be being planned, adding he had decided to go public out of concern that national security is at risk. No major damage: Transformers were shot at over 100 times, but no serious damage was sustained, nor was anyone injured . Cables cut: At least one suspect cut communications cables along this highway and another before shooting up the substation . Energy officials in America have been concerned for some time that the country's electric grid could be vulnerable to sabotage. The grid is made up of three systems serving different areas of the country. In the past small glitches, including trees hitting transmission lines, has wrought havoc on the system, resulting in widespread blackouts. In 2003 50 million people in eastern states and parts of Canada were left without power for days after a similar minor incident. Concerns also focus on the fact many of the most important parts of the network are out in the open, often in remote locations, protected by little more than cameras and high fences. Transmission power stations, like Metcalf, are vital to the grid, making it possible for electricity to move the vast distances needed to serve the entire country. The 64-year-old, who stepped down from his role last November, said he was involved in a series of high-level private briefings with federal agencies, Congress and the White House in the aftermath of the attack. Federal . officials took over the investigation from local law enforcement after . fears were raised that the attack was linked to the Boston marathon bombing a day earlier. But an FBI spokesman in San Francisco told the WSJ they do not believe last April's attack is the work of a terrorist group, adding that investigators are still examining the evidence. However some of Mr Wellinghoff's colleagues in the utility industry share his concerns. A former official at PG&E - who own the Metcalf power station - told an industry gathering in November that he fear the attack was a 'dress rehearsal' for a larger, more catastrophic incident. Retired vice president of transmission for PG&E, Mark Johnson, said: 'This wasn't an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation. 'This was an event that was well thought out, well planned and they targeted certain components.' When reached by the WSJ, Mr Johnson said he could not expand on his comments at the conference. PG&E spokesman Brian Swanson said the company take all incidents seriously but would not comment further for fear of giving valuable information to copycat groups. He said: 'We won't speculate about the motives (of the attack).' But he added the group have increased security measures in the wake of the incident. Lit up: The Santa Clara Sheriff's Office released this surveillance footage of the attack which shows the gunshots going off on the grounds of the PG&E facility . Sparks flew: No one was injured and the power didn't actually go out in the area because the state grid was able to maintain it but officials fear that it was a test run . In 2009 a report by the Energy Department said that 'physical damage' to certain parts of the grid, namely transformers, could render the system debilitated, resulting in prolonged blackouts. Mr. Wellinghoff said research by the FERC found that it would only take a small number of substations to be knocked out at once to destablise the grid to such an extent that a blackout would engulf most of the whole country. But Gerry Cauley, chief executive of the North America Electric Reliability Corp, a standards-setting group that reports to the FERC, disagreed. He said he believes the grid is more resilient. 'I don't want to downplay the scenario he describes,' Mr. Cauley said. 'I'll agree it's possible from a technical assessment.' But he said that even if several substations went down, the vast majority of people would have their power back in a few hours. A spokesman for Homeland Security said it is the responsibility of utilities companies to protect the grid. The department's role in an emergency is to connect federal agencies and local police and facilitate information sharing, the spokesman said.
As many as two gunmen stormed the PG&E Metcalf substation in California on April 16 last year .
Former chairman of the Federal Energy commission branded the attack 'the most significant domestic terrorist assault on the grid'
Jon Wellinghoff, who stepped down in November, was moved to speak publicly for fears over national security .
He and colleagues believe the attack was a 'dress rehearsal' to a much larger and more serious incident .
FBI investigating but say they do not believe terrorist group is responsible .
By . Lizzie Parry . Experts in the United States have warned an assault on a Californian power station in April could have been the 'dress rehearsal to a larger terrorist attack'. On April 16, last year, as many as two gunmen stormed the PG&E Metcalf substation in California after severing phone lines and firing several dozen rounds at transformers. At . least one person, maybe two, went down multiple manholes at the . facility in a San Jose suburb and cut fiber cables leading to the . substation. Attacked: The FBI is investigating an April attack on PG&E's Metcalf substation. Experts have warned the assault could have been a 'dress rehearsal for a larger attack' This knocked out 911 and landline service to the power station, as well as mobile phone service to the surrounding area. PG&E employees had no means to call for help when, at 1am, gunmen began their attack. More . than 100 rounds were fired from high-powered rifles at many . transformers – 10 were damaged in one area and three transformer banks . in another, a PG&E spokesperson said at the time. Cooling . oil leaking from at least one transformer bank caused transformers to . overheat and shut down. This led officials to warn locals to conserve . energy, but no major power outages occurred. Former chairman of the Federal Energy Regulatory Commission, Jon Wellinghoff, has today called the attack 'the most significant incident of domestic terrorism involving the grid that has ever occurred'. He told the Wall Street Journal he has become 'increasingly concerned' that a larger attack could be being planned, adding he had decided to go public out of concern that national security is at risk. No major damage: Transformers were shot at over 100 times, but no serious damage was sustained, nor was anyone injured . Cables cut: At least one suspect cut communications cables along this highway and another before shooting up the substation . Energy officials in America have been concerned for some time that the country's electric grid could be vulnerable to sabotage. The grid is made up of three systems serving different areas of the country. In the past small glitches, including trees hitting transmission lines, has wrought havoc on the system, resulting in widespread blackouts. In 2003 50 million people in eastern states and parts of Canada were left without power for days after a similar minor incident. Concerns also focus on the fact many of the most important parts of the network are out in the open, often in remote locations, protected by little more than cameras and high fences. Transmission power stations, like Metcalf, are vital to the grid, making it possible for electricity to move the vast distances needed to serve the entire country. The 64-year-old, who stepped down from his role last November, said he was involved in a series of high-level private briefings with federal agencies, Congress and the White House in the aftermath of the attack. Federal . officials took over the investigation from local law enforcement after . fears were raised that the attack was linked to the Boston marathon bombing a day earlier. But an FBI spokesman in San Francisco told the WSJ they do not believe last April's attack is the work of a terrorist group, adding that investigators are still examining the evidence. However some of Mr Wellinghoff's colleagues in the utility industry share his concerns. A former official at PG&E - who own the Metcalf power station - told an industry gathering in November that he fear the attack was a 'dress rehearsal' for a larger, more catastrophic incident. Retired vice president of transmission for PG&E, Mark Johnson, said: 'This wasn't an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation. 'This was an event that was well thought out, well planned and they targeted certain components.' When reached by the WSJ, Mr Johnson said he could not expand on his comments at the conference. PG&E spokesman Brian Swanson said the company take all incidents seriously but would not comment further for fear of giving valuable information to copycat groups. He said: 'We won't speculate about the motives (of the attack).' But he added the group have increased security measures in the wake of the incident. Lit up: The Santa Clara Sheriff's Office released this surveillance footage of the attack which shows the gunshots going off on the grounds of the PG&E facility . Sparks flew: No one was injured and the power didn't actually go out in the area because the state grid was able to maintain it but officials fear that it was a test run . In 2009 a report by the Energy Department said that 'physical damage' to certain parts of the grid, namely transformers, could render the system debilitated, resulting in prolonged blackouts. Mr. Wellinghoff said research by the FERC found that it would only take a small number of substations to be knocked out at once to destablise the grid to such an extent that a blackout would engulf most of the whole country. But Gerry Cauley, chief executive of the North America Electric Reliability Corp, a standards-setting group that reports to the FERC, disagreed. He said he believes the grid is more resilient. 'I don't want to downplay the scenario he describes,' Mr. Cauley said. 'I'll agree it's possible from a technical assessment.' But he said that even if several substations went down, the vast majority of people would have their power back in a few hours. A spokesman for Homeland Security said it is the responsibility of utilities companies to protect the grid. The department's role in an emergency is to connect federal agencies and local police and facilitate information sharing, the spokesman said.
rehearsal to a larger terrorist attack'. On April 16, last year, as many as two gunmen stormed the PG&E Metcalf substation in California after FBI is investigating an April attack on PG&E's Former chairman of the Federal Energy Regulatory attack 'the most significant incident of domestic stepped down from his role last November, said he not believe last April's attack is the work of a attack was a 'dress rehearsal' for a larger, more disagreed. He said he believes the grid is more