The US attorney general, John Ashcroft, told a press briefing on 8 October that warnings had been issued through the National Threat Warning System, a network used by the FBI to communicate with law enforcement agencies around the country.
As well as placing thousands of law enforcement officials on the highest state of alert, the FBI is also extending warnings to more than 27,000 corporate security officers at companies that own and operate systems such as telecommunications, banking and finance, railways, water supply, transportation and electric power facilities.
A spokesperson for the FBI's National Infrastructure Protection Centre said the warnings served to encourage a "heightened awareness for the security and safety of critical infrastructure systems in the aftermath of 11 September and especially since the beginning of US military strikes."
Intelligence officials have told members of the US Congress that the likelihood of further attacks, either physical or cyber, is virtually certain.
Security officials continue to express concern about the interdependent nature of critical infrastructure systems in the US, warning that failures in one sector could lead to serious problems affecting many, if not all, other sectors of the economy.
Although federal authorities and troops are being deployed to defend some critical facilities - such as nuclear power plants and water treatment facilities - the bulk of the national critical infrastructure facilities and networks are owned and operated by private companies.
"The federal government cannot post soldiers or police officers at the perimeters of telecommunications facilities or electric power plants to keep out digital attackers," said John Tritak, director of the US Commerce Department's Critical Infrastructure Assurance Office.
Meanwhile, President Bush hosted a swearing-in ceremony at the White House to appoint former Pennsylvania governor Tom Ridge as the new cabinet-level director of the Office of Homeland Security. Starting today, Ridge is responsible for coordinating and managing the US Government's response to terrorist attacks, including major terrorist-sponsored disruptions of critical cybersystems.
Ridge described the problem of defence as "immense", and said it would require an unprecedented level of cooperation between the government the private sector. "Everyone must play a part," he said. "It calls for a national effort."