India, April 27 -- For decades, the 1990 theft of 13 artworks from the Isabella Stewart Gardner Museum - now valued at more than $500 million - has remained unsolved.

It remains the largest art theft in history - far surpassing more recent museum thefts, including a daylight heist at the Louvre that involved far fewer works and was resolved more quickly.

In 2013, the FBI said it knew who was responsible for the Boston museum heist but declined to name them, fuelling speculation that persists today.

A former FBI agent who led the investigation for more than two decades is now offering the first detailed account of how investigators reached that conclusion - and publicly identifying the men he believes were involved.

In a new book, Geof...