After more than two years and a 455-page report they were unable to determine what the lyrics of the song were

From Jack Ely, Who Sang the Kingsmen’s ‘Louie Louie’, Dies at 71 by Sam Roberts.

High school and college students who thought they understood what Mr. Ely was singing traded transcripts of their meticulously researched translations of the lyrics. The F.B.I. began investigating after an Indiana parent wrote to Attorney General Robert F. Kennedy in 1964: “My daughter brought home a record of ‘LOUIE LOUIE’ and I, after reading that the record had been banned on the air because it was obscene, proceeded to try to decipher the jumble of words. The lyrics are so filthy that I cannot enclose them in this letter.”

The F.B.I. Laboratory’s efforts at decryption were less fruitful. After more than two years and a 455-page report, the bureau concluded that “three governmental agencies dropped their investigations because they were unable to determine what the lyrics of the song were, even after listening to the records at speeds ranging from 16 r.p.m. to 78 r.p.m.”

And here's the song.