FireEye revealed that its own systems were pierced by what it called "a nation with top-tier offensive capabilities". The company said hackers used "novel techniques" to make off with its own tool kit, which could be useful in mounting new attacks around the world.

FireEye said on Tuesday, moments after the stock market closed, that it had called in the FBI. The $3.5 billion company, which partly makes a living by identifying the culprits in some of the world's boldest breaches -- its clients have included Sony and Equifax -- declined to say explicitly which state was responsible.

However its description of the attack and the fact that the FBI has turned the case over to its Russia specialists, left little doubt who the lead suspects were and that they were after what the company calls "Red Team tools".

These are essentially digital tools that replicate the most sophisticated hacking tools in the world. FireEye uses the tools — with the permission of a client company or government agency -- to look for vulnerabilities in their systems. Most of the tools are based in a digital vault that FireEye closely guards.

It seems that the attack happened when FireEye’s attention was focused on securing the presidential election system. At a moment that the nation's public and private intelligence systems were seeking out breaches of voter registration systems or voting machines, it may have a been a good time for those Russian agencies, which were involved in the 2016 election breaches, to turn their sights on other targets.

The hack was the biggest known theft of cybersecurity tools since those of the National Security Agency were purloined in 2016 by a still-unidentified group that calls itself theShadowBrokers.