By day, Ana Montes was one of America’s most trusted intelligence experts — a senior analyst deep inside the Defense Intelligence Agency. She advised military leaders, handled classified information, and helped shape U.S. national security decisions.

But behind closed doors, investigators say she was living a second life.

For nearly two decades, Montes secretly worked for Cuban intelligence while operating from the heart of the United States government. No stolen briefcases. No dramatic car chases. Instead, prosecutors say she relied on memory, coded messages, shortwave radio transmissions, and quiet meetings that allowed one of the most devastating espionage operations in modern American history to unfold unnoticed.

How did a trusted government insider become a Cuban spy? What secrets did she hand over? And how did investigator ...