A whistle-blower said the tax agency found a message from 2017 in which Hunter Biden pressured a Chinese business partner by saying he was with his father, who was then out of office.

Mr. Shapley said that investigators had received the WhatsApp message in response to a search warrant. But the message was not among those that Mr. Biden’s lawyers handed over to federal prosecutors in response to grand jury subpoenas during the lengthy investigation run by the Justice Department with help from the I.R.S., according to a person familiar with the matter, nor was the message one that prosecutors pressed Mr. Biden’s lawyers to explain.