Mourning Courier and New-York Enquirer suggests that Sidney Rigdon wrote the BOM and Joseph fallaciously claimed to translate it.
“Mormon Religion— Clerical Ambition— Western New York.— The Mormonites Gone to Ohio,” Morning Courier and New-York Enquirer (New York City) 7, no. 1334 (September 1, 1831): 2
At last a printer in Palmyra undertook to print the translations of Joe Smith, Harris becoming responsible for the expense. They were called translations, but in fact and in truth they are believed to be the work of the Ex-Preacher from Ohio, who stood in the back ground and put forward Joe to father the new bible and the new faith. After the publication of the golden bible, they began to make converts rapidly.