Since much of Irving’s scam had taken place using the mail, postal inspectors began an investigation. They focused on letters supposedly written by Hughes. Although McGraw-Hill had hired their own experts to authenticate the handwriting, which they determined could be Hughes’, postal inspectors took another tact. As Inspector John Tarpey told a Toledo Blade reporter in 1973, “We tried to approach it from the other side. We wanted to see whether someone else could have forged the documents, and we picked Irving to begin with.” They had to get a government subpoena to force Irving to turn over samples of his handwriting. Tarpey noted that “The writing was different from the Hughes documents, but close examination shows that some characteristics and habits were the same. The T's were crossed the same, the I's were dotted the same, and some letters were broken up alike. We took the samples to our own handwriting experts who studied them for two days and confirmed that they were the same.”
Irving finally admitted on March 30, 1972, that he had forged the letters and faked the billionaire’s memoir. He pled guilty and was sentenced on July 16, 1972 and spent 17 months in prison.