The US Justice Department is investigating the finances of Hunter Biden, the son of US president-elect Joe Biden.
The investigation includes scrutinising some of his Chinese business dealings and other transactions, a person familiar with the matter has told the Associated Press.
Hunter Biden confirmed the existence of the investigation on Wednesday, saying he learned about it for the first time the previous day.
The revelations put a renewed spotlight on questions about Hunter Biden's financial history, which dogged his father's successful White House campaign and were a frequent target of President Donald Trump and his allies.
They also come at a politically delicate time for the president-elect, who is weighing his choice to lead an agency that is actively investigating his son.
The tax investigation was launched in 2018, the year before the elder Biden announced his candidacy for president.
"I take this matter very seriously but I am confident that a professional and objective review of these matters will demonstrate that I handled my affairs legally and appropriately, including with the benefit of professional tax advisors," Hunter Biden said in a statement.
Hunter Biden has a history of international affairs and business dealings in a number of countries.
Trump and his allies have accused him of profiting off his political connections, and have also raised unsubstantiated charges of corruption related to his work in Ukraine at the time his father was vice president and leading the Obama administration's dealings with the eastern European nation.
Late on Wednesday, Trump tweeted a quote from New York Post columnist Miranda Devine claiming, "10% of voters would have changed their vote if they knew about Hunter Biden."
It isn't clear which entities or business dealings might be tied up in the probe, though the person with knowledge of the matter said at least some of the focus was on his past work in China.
Federal investigators served a round of subpoenas on Tuesday, including one for Hunter Biden, according to another person familiar with the investigation.
Investigators did not reach out until recently because of Justice Department practice against taking overt investigative actions in the run-up to an election.Those familiar with the investigation wanted to remain anonymous when discussing the matter, the AP reported.
Biden is actively assembling his Cabinet, but has yet to name a nominee to lead the Justice Department. That person could ultimately have oversight of the investigation into the new president's son if it is still ongoing when Biden is sworn in on January 20.
The transition team said in a statement: "President-elect Biden is deeply proud of his son, who has fought through difficult challenges, including the vicious personal attacks of recent months, only to emerge stronger."
Biden's relationship with China
A New Yorker profile on Hunter Biden last year detailed some of his business work in China, including how he accompanied his father on a 2013 trip to Beijing, where he met with a business associate.
He also acknowledged having received a diamond from a Chinese energy tycoon interested in liquified natural gas projects.
He downplayed the idea that the gift could have been intended to affect his father's policy. He told the magazine he gave the diamond to an associate.
"What would they be bribing me for? My dad wasn't in office," he said.
Hunter Biden has been caught up in controversies before. While his father was vice president, Hunter joined the Naval Reserve and was discharged after testing positive for cocaine in his system, later revealing a year long struggle with addiction.
He also joined the board of Ukrainian energy company Burisma in 2014, sparking concerns about the perceptions of a conflict of interest given the elder Biden was deeply involved in US policy toward Ukraine.
An investigation by the Republican-led Senate did not identify any policies that were directly affected by Hunter Biden's work.