While the investigation is currently focused on seven 345kV transmission lines that failed in Ohio, the inquiry has widened to include other anomalies in the operation of the grid from the Midwest to the East Coast.

Three power plants and five high voltage transmission lines failed shortly before the outages. Michehl Gent, president of the North American Electric Reliability Council, identified the failed Ohio transmission lines as “the area where the cascading outages began.” However, he also cautioned that “any attempt on our part to identify the cause of the outages at this point would be speculative and premature.” Investigators are looking at several preblackout events, including one in Michigan. Gary Kitts, chief administrative officer of the Michigan Public Service Commission, said that after lines went dead in Ohio, Michigan power operators began seeing an unusually heavy flow of electricity come into the state from Indiana minutes before the blackout and then move on to Ohio. The surge was massive: 3700MWe, equal to 20% of Michigan’s daily power demand. “We’re trying to understand what the impact that much magnitude had on the system in Michigan,” Kitts said.

Patrick Wood, chairman of the Federal Energy Regulatory Commission, said that he expects a joint US-Canadian task force probing the outages will need weeks to get answers. Some of the variables that it will consider include: * Trouble outside Ohio before the failures. The organisation that runs the Midwestern power grid and FirstEnergy, which owns one of the power plants and three of the transmission lines that failed in the two hours before the outages, contend that unusual events happened across the entire Eastern US grid starting at noon on 14 August, including big swings in voltage and frequency. They argue that the Cleveland area failures could have been symptoms of earlier problems, not the prime cause of the outages.

* Human error. The reliability council said that a key question is whether grid technicians failed to move quickly enough to prevent problems in Ohio from spreading. Officials with the New England and mid Atlantic power grids said they took steps quickly around 4.10 pm on 14 August to shut down their connections to New York and the Midwest. These moves largely succeeded in helping prevent outages from spreading still further. The task force will consider whether PJM Interconnection and Independent System Operator New England, the grid operators for the Mid Atlantic and New England region, followed procedures better than Midwestern and New York technicians, or if they did a better job in maintaining and upgrading grid control technologies.

* Ontario demand. The events happened in the context of a major shift in power flows between the USA and Ontario. For most of the 1980s and 1990s, Ontario was an exporter of power to the USA. Recently, however, Ontario’s summer demand has risen, and its capacity has fallen as a result of several of its nuclear plants being out of service. Just before the outage, Ontario was importing 2000MWe from New York, Minnesota, Michigan and other Canadian provinces.

* The Lake Erie loop. Power to and from Ontario flows through a network known as the Lake Erie Loop, which runs from Niagara Falls to Detroit on both sides of the lake, and which has long been identified as a highly stressed, volatile system in need of upgrades.

* Terrorism. Although there is no evidence of sabotage, terrorism or the actions of a rogue grid technician as the cause, the task force will “not rule anything out,” according to Ellen Vanko, spokeswoman for NERC.

There is some evidence to suggest that many utilities had little or no warning of the cascading problem before their own facilities shut down. A timeline of power problems compiled by NERC showed power swings in eastern USA and Canada at 4.08pm, and a series of line failures in Michigan starting nine minutes later.

Independent system operators are tied together by a system of telephone hotlines. The timing and content of the conversations preceding the blackout remain unclear. Ontario has complained that it was given no warning of the problem. PJM Interconnection has claimed that it was given just 11 seconds warning, despite there having been unusual grid activity in the Midwest for some time before this. The Midwest power system recorded numerous voltage swings as early as noon on 14 August.

White House press secretary Scott McClellan reiterated president Bush’s support of energy legislation that would establish enforceable standards of reliability on the electricity transmission system.