The lead I.R.S. agent investigating whether Hunter Biden committed tax crimes told Congress his team uncovered evidence that Mr. Biden had invoked his father, who was then out of office, while pressing a potential Chinese business partner in 2017 to move ahead with a proposed energy deal, House Republicans said.

In testimony made public on Thursday, Gary Shapley, an I.R.S. agent since 2009 who supervised the tax agency’s investigation into Hunter Biden, said his team used a search warrant to obtain a July 30, 2017, WhatsApp message from Mr. Biden to Henry Zhao, a Chinese businessman.

In a summary of the message, provided to the House Ways and Means Committee by Mr. Shapley, Mr. Biden told Mr. Zhao that he was sitting with his father and that “we would like to understand why the commitment made has not been fulfilled.”

“Tell the director that I would like to resolve this now before it gets out of hand, and now means tonight,” Mr. Biden wrote, referring to other participants in the proposed deal. “And, Z, if I get a call or text from anyone involved in this other than you, Zhang, or the chairman, I will make certain that between the man sitting next to me and every person he knows and my ability to forever hold a grudge that you will regret not following my direction.”

Taken at face value, the message would undercut President Biden’s longstanding claims that he had nothing to do with his son’s international business deals.

Coming days after Hunter Biden agreed to plead guilty to two misdemeanor charges of failing to file his taxes on time in 2017 and 2018, the release of the information underscored the determination of House Republicans to continue to try to tie his father to his business dealings and suggest that they were corrupt.

But it was not immediately clear whether Hunter Biden had been with his father when he sent the message or what his father — then a private citizen, having finished his term as vice president six months earlier — knew about his son’s negotiations with his potential Chinese partners.

It is also not clear whether Mr. Biden was using his father’s name without his knowledge to extract money in a business deal. Mr. Shapley, in fact, also told Congress that his investigation had uncovered some evidence that some of the claims of the elder Mr. Biden’s involvement were mere “wishful thinking.”

He told of an interview conducted with Hunter Biden’s business associate Rob Walker, who told investigators that it was “projection” that former Vice President Biden would get involved in their business ventures.

“I certainly never was thinking at any time the V.P. was a part of anything we were doing,” Mr. Walker said, according to Mr. Shapley.

Hunter Biden at the time was addicted to crack cocaine, engaging in tawdry and self-destructive behavior, and facing financial pressures. He had spent years pursuing ventures that raised ethical concerns about their intersection with his father’s career, especially during the eight years the elder Mr. Biden spent as vice president. And the death of his brother, Beau, in 2015 had contributed to his downward spiral.

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.

It was also not among the trove of messages that have surfaced from a laptop Mr. Biden left at a repair shop in Delaware and was later made available to journalists.

The WhatsApp message was among a batch of documents released by the Ways and Means Committee along with the transcripts of interviews with Mr. Shapley and a second I.R.S. investigator whose name was redacted.

The two investigators, one of whom described himself as a Democrat, told Congress of a lengthy period of strife between them and others involved in the investigation. They said a particular prosecutor at the Justice Department blocked some of their efforts and communicated too much information to Hunter Biden’s legal team.

Mr. Shapley suggested that I.R.S. investigators believed there were grounds to charge Mr. Biden with more serious crimes than he ultimately agreed to plead guilty to as part of his deal with the Justice Department. Mr. Shapley told the committee that he was “alleging, with evidence, that D.O.J. provided preferential treatment, slow-walked the investigation, did nothing to avoid obvious conflicts of interest in this investigation.”

House Republicans sought to portray the testimony as further evidence that Hunter Biden had gotten what they call a sweetheart deal from the Justice Department, even though his agreement to plead guilty to two misdemeanor charges appeared in line with how other first-time, nonviolent offenders were typically treated. Mr. Biden paid his back taxes and penalties in 2021.

In a letter to Congress this month, David C. Weiss, the U.S. attorney from Delaware who charged Hunter Biden over his tax issues and a separate gun purchase, said he had “ultimate authority” over the case. Mr. Weiss was appointed by President Donald J. Trump and was kept on to complete the investigation under President Biden.

A Justice Department spokesman reiterated on Thursday that Mr. Weiss had “responsibility for deciding where, when and whether to file charges as he deems appropriate,” and did not need any further approval to do so. The White House said President Biden had no involvement in the investigation and that it had been conducted “free from any political interference.”

“Hunter Biden profited off his father’s name. He didn’t report millions of dollars in income from foreign sources,” said Representative Jason Smith, Republican of Missouri and the chairman of the Ways and Means committee. “Any ordinary American caught doing that would have faced severe consequences. Hunter Biden didn’t. Why not?”

Representative Richard E. Neal of Massachusetts, the top Democrat on the Ways and Means Committee, accused Republicans of rushing out piecemeal findings from their work without conducting a complete investigation.

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