As federal police agents unraveled a Kremlin spying operation in Brazil, they confronted a mystery: How had so many deep-cover Russian spies managed to obtain seemingly authentic Brazilian birth certificates?

The police expected to find that the Russians had forged the documents or bribed municipal officials to create them and slip them into the registry as if they were from the 1980s and ’90s.

But when the forensic report came back in April, according to a senior Brazilian official, the analysis suggested something else entirely. The documents did not appear forged. And, most surprising, they weren’t even new.

Brazilian counterintelligence officers are now considering a more audacious possibility, one with echoes of the Cold War. Investigators suspect that K.G.B. operatives, working undercover in Brazil during the last years of the Soviet Union, may have filed birth certificates in the names of fictitious newborns — hoping that a future generation of spies would someday claim them and continue the fight against the West.

If true, it would represent an extraordinary level of foresight and mission commitment by intelligence officers during a time of great upheaval and unpredictability in the world. By the late 1980s, the Communist bloc had begun to crumble, along with the ideological divisions that had defined global politics — and the mission of Moscow’s spies — for decades.

Almost overnight, the K.G.B., once an unparalleled force in global affairs, was deprived of its central purpose, conflict with the West, and would soon be disbanded entirely.