In notes accompanying the sale of his original manuscript, which was auctioned for $US1.2 million ($1.57 million) in New York on Tuesday, he described the ballad as a morality song that charted the decline of America and its loss of innocence.

While details of the lyrics have long been guessed - taking as its starting point his reaction to hearing in 1959 that Buddy Holly had died in a plane crash with Ritchie Valens and the Big Bopper on "the day the music died" - few have confidently decoded the riddle in its complex verses and scattergun cultural references.

Now he has explained almost all. "Basically in American Pie things are heading in the wrong direction," he said in an interview published in Christie's auction catalogue.

"It is becoming less idyllic. I don't know whether you consider that wrong or right but it is a morality song in a sense."

Eighteen pages of handwritten notes and typed drafts that charted the evolution of the song's lyrics were sold to an unnamed buyer. 

American Pie made number one in the US and is frequently cited as an important piece of cultural history.

McLean, now 69, has always avoided answering questions about its meaning, other than to joke: "It means I don't ever have to work again if I don't want to."

He initially said he was selling the manuscript on a whim, but in the catalogue added that he wanted to inspire young songwriters to think about every word in a song.

He said the lyrics reflected the way the world had declined in the years since it was written.

"I was around in 1970 and now I am around in 2015... there is no poetry and very little romance in anything any more, so it is really like the last phase of American Pie," he said.

The final verse of the recorded version describes a bleak America: the music has gone and even the Father, Son and Holy Ghost are heading for the coast.

But his notes offer the tantalising prospect of a lost redemptive verse. It describes falling to his knees to pray: "And I promised to give all I have to give, If only he would make the music live again."

His prayers are answered, "and the music lived again" or "was reborn" depending on which version you are reading.

Somewhere it was lost and the bleak epic of a destroyed land was left intact. The different coloured pieces of manuscript also highlight the way the song was written over an extended period, in different notebooks, in 1970 and 1971.

For a long time, all McLean had was the opening line and the hook, "Bye, bye Miss American Pie". He said: "So I was into this and thinking about it all the time in my own head while I was doing things like normal people do and one day I was in Cold Spring, NY, and I was in town and I believe I went into the shopping area and I went into the pharmacy and all of a sudden it hit me." One chunk of the song came quickly followed by a pause of several months before he managed to write five verses in an hour in Philadelphia.

Although McLean declined to run through all the cultural references, the catalogue said it was probably safe to conclude that references to the king meant Elvis Presley, Helter Skelter referred to the Charles Manson murders, and "the jester on the sidelines in a cast" was Bob Dylan. But that still leaves plenty of other details up for debate.

The Telegraph, London