Neither a cyber-attack nor unpredictable wind power generation were behind a power outage that left nearly one million people in England and Wales in the dark Friday.

The outage stranded trains, disabled traffic signals and left thousands of homes across much of the England and Wales without power, including large parts of London.

National Grid said two generators, one gas-powered and one-wind powered, conked out Friday afternoon. Automatic safety systems shut off the juice to other spots after the generators went down.

Ofgem, the power regulator in Britain, demanded an urgent investigation and said enforcement action could come once the cause was determined.