Vasili Mitrokhin, the former KGB archivist who gave British authorities documents revealing aspects of Soviet espionage activities—including those of long-standing Latvian-British “granny spy” Melita Norwood—has died, according to British media reports.

The British government announced the death of Mitrokhin on Jan. 30, the daily newspaper The Guardian reported. Mitrokhin defected in 1992, turning to the British embassy in Latvia after his offer reportedly was rejected by American officials. Since then, he had been living under a false name in the United Kingdom the newspaper said.

Among revelations in his documents, which led to the 1999 publication of The Mitrokhin Archive, was that Norwood had quietly spied for the Soviets for 40 years while working as secretary for the British Non-Ferrous Metals Research Association. She reportedly had passed on secrets regarding the West’s work on the atom bomb.

Norwood was born in 1912 as Melita Sirnis. Her father was a Latvian immigrant. An anarchist and communist, he died when his daughter was six years old.