A company based in The Hague said it had found Internet access to the control systems of more than 8,500 wind and solar parks in 35 European countries. It said that, in some cases, the systems could allow someone to switch a wind turbine on or off remotely, potentially disrupting power supplies to hundreds of thousands of households. The account said criminals or hostile states could have used such access to shut down the flow of electricity, but it did not say that an outage had occurred.