We may never know the true origin of the hoax. However, we do know how it was brought into public knowledge.
In 1969 Russell Gibb, a radio Disc Jockey in Detroit, announced that Paul McCartney was dead. His proof: evidence strewn throughout the Beatles' songs, movies, and album artwork. Newspaper and television reporters picked up the story and the news quickly spread across America.
THE STORY OF PAUL'S DEATH
The story states that Paul McCartney was involved in a car accident. Apparently "he hadn't noticed that the lights had changed." The accident occurred at 5 a.m. on a Wednesday morning. Cause of death was massive head trauma. So severe were his wounds that dental records were useless for identifying the body.
No accident report was made public due to pending investigation of the occupant. Beatles' insiders learned it was Paul, but kept silent. Understanding the ramifications of such news, the remaining Beatles hatched a cover-up.
A Paul McCartney look alike contest was held. William Campbell won first place, but the results were never announced. Campbell's prize was to be made a clone of Paul for photos, videos, movies, etc. Plastic surgery was used to smooth out the minor differences. They failed to fix a scar on Campbell's upper lip - this is how you can tell authentic McCartney photos from the Campbell ones.
Was it a practical joke, a cruel hoax, or simply all coincidence? Most of the clues are very ambiguous. The certainly do not offer concrete evidence as to whether or not they were planted or simply coincidental. Most of the clues can be refuted wither way. Who was behind the whole thing? Nobody knows for sure. I am certain that each Beatle has publicly denied having anything to do with the hoax.