It remains one of the greatest unsolved mysteries in the United States, a startling crime that captured the American imagination, inspiring songs, movies, TV shows and books.

In 1971, a man who called himself Dan Cooper hijacked a passenger plane from Oregon to Seattle where he freed the 36 passengers in exchange for $200,000 in cash. As the nearly empty flight took off again, flying south, he parachuted out of the airplane with the ransom, and was never seen again.

But after 45 years in which hundreds of leads were probed and discarded, the F.B.I. said this week it was no longer actively pursuing what it called one of the longest and most exhaustive investigations in its history.

Who was D.B. Cooper?

No one knows. Or someone does, but is not telling. The F.B.I. has described him as a “nondescript” man. He appeared to be in his mid-40s, which if true would make him about 90 years old by now. As the caper became widely known, he was referred to as “D.B. Cooper” in media reports.