Attorney General Merrick B. Garland defended the Justice Department’s five-year investigation of Hunter Biden on Friday, forcefully rebutting claims promoted by House Republicans that he blocked federal prosecutors in Delaware from expanding the inquiry to encompass a greater range of crimes.

On Tuesday, the House Ways and Means Committee released testimony from two I.R.S. officials who said that David C. Weiss, the U.S. attorney for Delaware who has overseen the inquiry, told them that Justice Department officials prevented him from bringing cases in Washington, D.C., and California.

The I.R.S. officials also claimed in their testimony that Mr. Weiss told them that he was rebuffed in his request to be appointed a special counsel, which would have allowed him to file a detailed report of his findings that could potentially be made public once the probe is ended.

Mr. Garland denied both assertions during a news conference at the department’s headquarters, saying he had given Mr. Weiss “complete authority” to “continue his investigation and to make a decision to prosecute any way in which he wanted to and in any district in which he wanted to.”

He scoffed at the idea that Mr. Weiss, a veteran prosecutor would have wanted to be designated as a special counsel, given the authority he already possessed as a U.S. attorney.

Mr. Weiss was named U.S. attorney in Delaware by President Donald J. Trump and eventually took control of the investigations into Hunter Biden, President Biden’s son, during the Trump administration. He was kept on by Mr. Garland to allow him to complete the investigation. Mr. Garland and Mr. Weiss have both made public statements that Mr. Weiss had full authority over the case.

“The only person who has the authority to make someone a special counsel, or refuse to make them a special counsel, is the attorney general,” he said, adding: “Mr. Weiss never made that request.”

Mr. Garland said he would not stop Mr. Weiss from testifying before Congress once the case was closed.

Mr. Weiss had no comment. But in announcing the agreement in the Hunter Biden case on Tuesday, he said that the investigation was “ongoing” — to the surprise of all parties.

It is not clear what he meant, but the declaration will have a near-term chilling effect on greater disclosure, effectively preventing Mr. Weiss from speaking publicly about the investigation until it is officially closed.

The release of the transcripts by the House committee came 48 hours after Mr. Weiss announced that Mr. Biden would plead guilty to two misdemeanor charges of failing to file his taxes on time and would enter a pretrial diversion program intended to allow him to avert prosecution on a separate gun charge.

House Republicans sought to portray the testimony by the I.R.S. officials as evidence that Hunter Biden had gotten a sweetheart deal from the Justice Department and that the department had been subject to political influence.

“If the federal government is not treating all taxpayers equally, or if it is changing the rules to engineer a preferred outcome, Congress has a duty to ask why and to hold agencies accountable and consider appropriate legislative action,” Representative Jason Smith, Republican of Missouri, chairman of House Ways and Means Committee, said in a statement on Thursday.

In the testimony released by the committee on Thursday, 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 a summary of a WhatsApp message provided to the committee, Mr. Biden told the business partner 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.”

It is not clear whether Mr. Biden’s father, then six months out of office after serving as vice president for two terms, was actually present for the call or was involved in any substantive way in the business activities of his son, who at the time was addicted to crack cocaine and having financial problems.

Christopher Clark, Hunter Biden’s lawyer in dealing with the Justice Department, said in a statement on Friday: “The D.O.J. investigation covered a period which was a time of turmoil and addiction for my client. Any verifiable words or actions of my client in the midst of a horrible addiction are solely his own and have no connection to anyone in his family.”

Mr. Clark said the assertions cited by the Ways and Means Committee came from a “very biased” witness.

“It is dangerously misleading to make any conclusions or inferences based on this document,” Mr. Clark said.