A former FBI agent is offering the first detailed account of how investigators identified the people believed to have carried out the 1990 Isabella Stewart Gardner Museum heist. In a new book, Geoffrey Kelly argues the theft was the work of a loose network of Boston-area criminals, not international masterminds, and traces how the stolen art moved through violent criminal circles. His account sheds light on key suspects, investigative missteps and lingering questions, including the actions of a security guard that night. More than three decades later, the paintings remain missing — artworks Kelly calls the “perfect fugitives.”