Former FBI agent Robert Hanssen, who was convicted of spying for Russia in notorious case, dies in prison

Formally named the Main Directorate of the General Staff of the Armed Forces, the agency is almost universally referred to by its former acronym GRU.

It is the most shadowy of Russia's secret services. When its previous director Igor Sergun died in 2016, the Kremlin announcement was so terse that it gave neither the date, cause or place of death.

Britain claims that two GRU agents carried out this spring's attack with the nerve agent Novichok on Sergei Skripal, a former GRU officer who became a British double agent, and his daughter. Both survived the poisoning in the city of Salisbury, but three months later two area residents were sickened by the same nerve agent, one of them fatally — it is believed they found the discarded bottle that had carried the Skripals' poison.

This week's claim came less than two months after the U.S. indicted 12 alleged GRU agents for hacking into the Hillary Clinton presidential campaign and the Democratic Party and releasing tens of thousands of private communications, part of a sweeping conspiracy by the Kremlin to meddle in the 2016 U.S. election.

The GRU is one arm of Russia's extensive security and intelligence apparatus, which also includes the Foreign Intelligence Service, known as the SVR, and the Federal Security Service, or FSB, which conducts domestic intelligence and counterintelligence. The SVR and FSB were spun off from the KGB after the collapse of the Soviet Union. A former KGB agent, Vladimir Putin ran the FSB before ascending to the presidency.

And as president, Putin names the top brass in the GRU. Of all the agencies, the FSB looms largest in Russians' minds because it hunts domestic threats. The GRU, created under Soviet founder Vladimir Lenin, has a more ruthless reputation, but focuses its energies on foreign threats.