A US justice department investigator leading an inquiry into US President Joe Biden's son has rejected claims of political meddling in the case during highly unusual testimony to Congress.
Special Counsel David Weiss told the House of Representatives Judiciary Committee that at no time has his probe been blocked by the justice department.
Hunter Biden, 53, is charged with owning a gun while using illegal drugs.
Two whistleblowers have alleged improper interference in the probe.
Mr Weiss's evidence on Tuesday behind closed doors is the first time a special counsel has testified to Congress in the middle of such an inquiry.
According to a written copy of his opening remarks reported by media, the US Attorney for Delaware told the panel: "I am, and have been, the decision-maker on this case. I do not, however, make these decisions in a vacuum.
"I am bound by federal law, the principles of federal prosecution and DoJ [Department of Justice] guidelines."
His statement added: "At no time was I blocked, or otherwise prevented from pursuing charges or taking the steps necessary in the investigation by other United States Attorneys, the Tax Division or anyone else at the Department of Justice."
Republicans have cited allegations from two agents with the Internal Revenue Service, the American tax authority, who testified in July that the justice department had hampered Mr Weiss's investigation into the president's son.
US Attorney General Merrick Garland, who oversees the justice department, testified in March that Mr Weiss had authority to act independently.
Although Mr Weiss was originally appointed by former President Donald Trump, Republicans have said the special counsel offered Hunter Biden a "sweetheart deal" free of jail time for tax evasion and gun charges.
Then-House Speaker Kevin McCarthy said the agreement was evidence of a "two-tiered" justice system and James Comer, Republican chair of the House Oversight Committee, called it "a slap on the wrist".
The plea deal unravelled in court over the summer. Hunter Biden was indicted anew in September on three federal gun charges for possessing a firearm while using a narcotic and falsifying documents when making a gun purchase.
Prosecutors are also investigating Hunter Biden's foreign business dealings to see whether he violated any laws such as government lobbying.
A second special counsel investigation, led by Robert Hur, is investigating whether President Biden mishandled classified documents from his time as vice-president.
The White House said in October that the president had been interviewed over two days for that investigation.
Hunter Biden's business dealings have been central to House Republicans' inquiry into whether the president should be impeached. The White House says the probe is "based on lies".