The federal investigation into President Joe Biden's son, Hunter Biden, over his tax affairs has intensified in recent weeks, sources familiar with the matter tell ABC News.
An increasing number of witnesses have appeared before a grand jury impaneled in Wilmington, Delaware, in recent months, the sources said, and have been asked about payments Hunter Biden received while serving on the board of directors of Ukrainian natural gas company Burisma, in addition to other questions about how Biden paid off tax obligations in recent years.
The U.S. Attorney's Office for Delaware, which has been leading the investigation, is expected to hear from more witnesses in the coming weeks, sources told ABC News.
Sources say that prosecutors have not made a decision yet on whether criminal charges against Biden are warranted, and that there is debate about whether Biden's admitted past drug abuse could serve to undermine a criminal case over his financial decision-making.
A spokesperson for the DA's office declined to comment to ABC News, as did a spokesperson for the Justice Department.
Chris Clark, an attorney for Hunter Biden, did not respond to a request for comment. The developments in the probe were first reported by The New York Times.
Hunter Biden confirmed the existence of the investigation in December 2020, shortly after his father won the presidency. Since taking office, President Biden has repeatedly pledged to uphold the independence of the Justice Department and not interfere in its work. The tax probe is being led by U.S. Attorney Dan Weiss, a Trump appointee who remained on the job after Biden was sworn in.
In a statement released through the Biden-Harris transition team in December 2020, Hunter Biden said that he and his attorney had learned of the investigation just days earlier, and that he was confident that he had handled his tax affairs "legally and appropriately."
ABC News has previously reported that the investigation began in 2018 -- more than a year before Biden announced his presidential run. Investigators paused their work ahead of the election and waited until after votes were cast to notify Hunter Biden's legal team of the probe, a source told ABC News at the time.
Ahead of the 2020 election, political foes of then-candidate Joe Biden -- including then-President Donald Trump -- focused on Hunter Biden's work as a board member for Burisma as well as a series of ill-fated investment endeavors in China. Trump and his allies sought unsuccessfully to characterize Hunter Biden's foreign business dealings over the past decade as evidence of Biden family corruption.
"In retrospect, look, I think that it was poor judgment on my part," Hunter Biden told ABC News Anchor Amy Robach in October 2019, regarding the impact of his business dealings on his father's political career. "Is that I think that it was poor judgment because I don't believe now, when I look back on it -- I know that there was -- did nothing wrong at all. However, was it poor judgment to be in the middle of something that is ... a swamp in -- in -- in many ways? Yeah."
While government watchdogs have broadly taken issue with the ethical implications of Hunter Biden's foreign business dealings, the federal probe marks the first possible legal threat tied to his overseas work.
Sources told ABC News that prosecutors in Delaware are investigating the tax implications of Hunter Biden's work in both China and Ukraine, among other business endeavors.
ABC News' Mike Levine contributed to this report.