Hackers affiliated to Al-Qaeda may be planning cyber attacks on nuclear power plants, dams or other critical structures.

According to a report in the The Washington Post, an FBI investigation of suspicious surveillance of key computers discovered "multiple casings of sites" nationwide.

Quoting a Defense Department summary of the probe, the newspaper said hackers routed through telecommunications switches in Saudi Arabia, Indonesia and Pakistan. They did this to study emergency telephone systems, electrical generation and transmission, water storage and distribution, nuclear power plants and gas facilities.

Some of the mysterious computer probes targeted a class of digital devices that allow remote control of such things as fire dispatch services and pipelines.

Information about those devices and hacking them were found on Al-Qaeda computers seized by US forces in Afghanistan.

The computer logs found show that Al-Qaeda operators spent time on sites that offer software and programming instructions for the digital switches running power, water, transport and communications grids.

The newspaper also said that some Al-Qaeda prisoners have told their US interrogators about intentions, in general terms, to use those tools.