Now that the election is over, the investigation is entering a new phase. Federal prosecutors in Delaware, working with the IRS Criminal Investigation agency and the FBI, are taking overt steps such as issuing subpoenas and seeking interviews, the person with knowledge said.
Activity in the investigation had been largely dormant in recent months due to Justice Department guidelines prohibiting overt actions that could affect an election, the person said.
CNN contacted Biden's attorney and the campaign this week seeking comment about the investigation. On Wednesday, they released a statement acknowledging the probe.
"I learned yesterday for the first time that the U.S. Attorney's Office in Delaware advised my legal counsel, also yesterday, that they are investigating my tax affairs. 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.
Investigators have been examining multiple financial issues, including whether Hunter Biden and his associates violated tax and money laundering laws in business dealings in foreign countries, principally China, according to two people briefed on the probe.
Some of those transactions involved people who the FBI believe sparked counterintelligence concerns, a common issue when dealing with Chinese business, according to another source.
The investigation began as early as 2018, predating the arrival of William Barr as US attorney general, two people briefed on the investigation said. The existence of the probe will present an immediate test of Biden's promise to maintain the independence of the Justice Department.
Sinclair Broadcast Group reported in October that the FBI had opened a criminal investigation into Hunter Biden. CNN has learned new details about the scope of the probe, including that it is focused on China.
Hints of the investigation emerged after President Donald Trump's personal attorney, Rudy Giuliani, helped orchestrate news stories centered on a laptop purported to belong to Hunter Biden and said to include his business documents and other personal material.
The FBI took possession of the laptop in late 2019, according to a computer repairman in Delaware who showed reporters a copy of a subpoena. The subpoena is real, according to people briefed on the matter, but the FBI and prosecutors in Delaware have refused to confirm the existence of the investigation.
It's unclear whether the laptop's contents are relevant to the ongoing federal probe and whether investigators can even use them, given potential chain of custody requirements for evidence.
CNN has previously reported that at least some of the information Giuliani claims came from the laptop appears similar to information that was being shared by others last year in Ukraine, according to one witness who the FBI has approached for information. Giuliani's efforts to dig up dirt on the Bidens in
Ukraine last year were at the heart of Trump's impeachment. The current investigation into Hunter Biden appears to predate those efforts.
The involvement of Trump lawyers in the published news stories on the purported Biden laptop led to a flurry of activity by Republican lawmakers and the Trump campaign looking to amplify allegations of corruption ahead of the election. The FBI met with a former Hunter Biden business associate who publicly made claims about potential wrongdoing. The FBI issued a statement at the time saying that "as a general matter when contacted, the FBI reviews information from the public for consideration of any violations of applicable federal laws."
The political implications of the probe already have been an issue in recent months for Barr, who was under public pressure from Trump to make Hunter Biden's business activities an election issue. To date, the investigation doesn't involve any allegations of wrongdoing by the President-elect, according to the two main sources briefed on the matter. Barr has said publicly the elder Biden was not under investigation, despite a public campaign by Trump urging Barr to announce a corruption probe of both Bidens.
Investigators appear to be focused on Hunter Biden's business activities connected to China. Some of his business dealings in China are publicly known through interviews and documents released in September by Senate Republicans on the Finance Committee and the Committee on Homeland Security and Government Affairs.
After his father left office in 2017, Hunter Biden worked on securing a deal with CEFC China Energy to invest in US energy projects, according to documents released by Republicans.
At least one of the matters investigators have examined is a 2017 gift of a 2.8-carat diamond that Hunter Biden received from CEFC's founder and former chairman Ye Jianming after a Miami business meeting, one person briefed on the matter said. In 2019, Biden provided the New Yorker magazine an account of receiving the diamond from Ye. He told the New Yorker that he felt uncomfortable receiving the diamond and gave it to other associates, and that he doesn't know what they did with it. "I knew it wasn't a good idea to take it. I just felt like it was weird," he said in his New Yorker interview.
Hunter Biden told the New Yorker that the CEFC deal fell through, and that he didn't consider Ye to be a shady character. Chinese authorities later detained Ye, amid Chinese media reports of corruption allegations against him.
In the magazine interview, Hunter Biden went public with his account of his personal struggles, including substance abuse and the breakup of his marriage.
During his divorce proceedings, attorneys for his ex-wife cited a diamond that Biden had received and suggested it was worth $80,000. Biden told the New Yorker it was worth about $10,000.
Receiving such a gift could have potential tax implications for Biden and his account to the New Yorker doesn't describe how he dealt with the issue thereafter. Sources tell CNN that investigators have been looking into whether Biden properly reported his income for tax purposes over a period of time.
While the diamond was initially part of the inquiry, it's unclear whether the gift remains part of the focus of investigators.
Hunter Biden later briefly acted as a lawyer to represent Patrick Ho, who ran an organization backed by CEFC, and who was convicted in 2018 of paying millions of dollars in bribes to officials in Chad and Uganda to benefit CEFC energy projects in those countries. Ho was sentenced to a three-year federal prison term. He has since been released and is back in Hong Kong, his attorney said.
Hunter Biden had been involved in an earlier Chinese business venture that drew concerns in the Obama White House and Joe Biden's staff, according to the New Yorker. In 2013, Hunter Biden became involved with US and Chinese partners who were creating an investment fund called BHR Partners for deals outside China. Hunter Biden was an unpaid member of the BHR board and took an equity stake after his father left office, the New Yorker reported.
In an interview with CNN's Jake Tapper last week, President-elect Biden pledged to maintain an independent Justice Department free of political influence.
"I'm not going to be telling them what they have to do and don't have to do. I'm not going to be saying, go prosecute A, B or C," Biden said.
"That's not the role -- it's not my Justice Department. It's the people's Justice Department.
"The person or persons I pick to run that department are going to be people who are going to have the independent capacity to decide who gets prosecuted and who doesn't," said Biden.
Republicans in Congress are almost certain to seize on the existence of the probe to further their argument that Hunter Biden's activities in China are a sign of his father's conflict of interest when it comes to dealing with foreign policy with Beijing.
It's also likely going to be an issue that Biden's attorney general nominee will have to face during a Senate confirmation hearing.
Trump faced a similar, but much thornier, problem after taking office in 2017 when his then-attorney general, Jeff Sessions, quickly had to recuse himself from overseeing the federal investigation of the Trump campaign and possible links to Russian interference in the 2016 election.
Sessions was part of the President's election campaign and he held meetings with the then-Russian ambassador, which became part of the FBI's investigation.
Biden's likely attorney general candidates aren't known to have such entanglements.