Another week, another wave of cyber alarm in America. On Wednesday both the New York Stock Exchange and United Airlines suspended activity for several hours due to mysterious computing problems, while the Wall Street Journal’s website briefly went down. All three insisted that the outages reflected technical hitches, not malicious attack. But many are anxious after past assaults on mighty American companies and agencies.

In February Anthem, an insurance company, revealed that cyber hackers had stolen information on 80m customers. The Washington-based Office of Personnel Management said cyber hackers had taken data on millions of federal employees. Companies ranging from retailers to banks have been attacked, too.

On Wednesday — just as the NYSE was frozen — Cambridge university and Lloyds insurance group released a report suggesting that if a cyber assault breached America’s electrical grid, this could create $1tn dollars of damage. A few minutes later, James Comey, the FBI director, told Congress that it is struggling to crack encryption tools used by jihadis. In May, Mr Comey said Islamic terrorists were “waking up” to the idea of using malware to attack critical infrastructure. It is scary stuff.