A Justice Department investigation into Hunter Biden's business activities has gained steam in recent months, with a flurry of witnesses providing testimony to federal investigators and more expected to provide interviews in the coming weeks, according to multiple sources familiar with the matter.

The probe, led by the US Attorney in Wilmington, Delaware, began as early as 2018 and concerns multiple financial and business activities in foreign countries dating to when Biden's father was vice president. Investigators have examined whether Hunter Biden and some of his associates violated money laundering, tax and foreign lobbying laws, as well as firearm and other regulations, multiple sources said.

To do so, law enforcement has gathered information from lobbyists connected to Hunter Biden, from his business partners, and from others who've observed his financial engagements, including a woman with whom he had a child.

Hunter Biden has not been charged with any crimes and has denied any wrongdoing. His father, President Joe Biden is not being investigated as part of the probe of his son's business activities, according to sources who have been briefed.

But the ongoing investigation has persistently raised questions about the ethics and behavior of the President's son and fueled right-wing political attacks. In 2019, then-President Donald Trump pressured Ukraine to investigate the Bidens over Hunter's work for Burisma, a Ukrainian energy company, eventually leading to Trump's first impeachment.

In 2020, 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 potentially salacious materials. CNN previously reported that 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.

Activity in the investigation has ebbed and flowed for years -- with coronavirus disruptions and pausing around the 2020 election -- and in some instances, investigators have not followed up for months after making initial outreach to possible witnesses.

But in recent months, investigative activity in the Biden probe has intensified along with discussions among Justice Department officials about the strength of the case, and whether more work is needed before seeking a decision on possible charges, according to people briefed on the matter who spoke on condition of anonymity to discuss the ongoing investigation.

Those discussions have involved investigators from the FBI and IRS Criminal Investigation agency and prosecutors in Delaware and at Justice Department headquarters, one person briefed on the matter said. Hunter Biden has publicly discussed his own substance abuse struggles, and some Justice officials have debated whether his open discussions of his past drug use could potentially weaken their case should they bring one.

Some officials have noted that Biden could argue he wasn't aware of wrongdoing because he was on drugs, the source said. Others have countered that Biden's own public accounts of his recovery show he was fully responsible for actions now under scrutiny, according to the person briefed on the matter.

Investigators have at various times inquired about multiple facets of Hunter Biden's life -- initially focusing on tax issues and money transfers related to business activities in China, according to multiple people familiar with the probe. They're also examining Biden's role while on the board of the Ukrainian energy company Burisma years ago, the sources said.

Biden has told associates he paid outstanding tax bills, and public records show more than $450,000 in state liens in Washington, DC, were lifted in 2020, indicating those liabilities were likely paid off.

But those payments haven't resolved his legal issues. Investigators have examined the source of funds to pay the tax bills.

The gun incident

Prosecutors also have examined a 2018 incident in which a firearm owned by Hunter Biden ended up tossed by his then-girlfriend into a dumpster in Wilmington, a person briefed on the matter said. Biden described in media interviews last year that he was addicted to drugs, which raised the possibility he broke federal law when he bought the firearm.

Federal law prohibits firearms purchases by anyone who uses or is addicted to illegal drugs. It's unclear whether the gun incident remains an active part of the investigation.

Biden has denied wrongdoing in his business activities. In late 2020 after being notified by the Delaware US attorney about the investigation, Hunter Biden said in a statement issued by his father's presidential transition office: "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's attorney, Christopher Clark, did not respond to multiple requests for comment in recent days.

The Delaware US Attorney's Office and the Justice Department declined to comment.

In recent months, Biden has tried to turn a page, publishing a memoir and debuting his work as an artist with shows in Los Angeles and New York. But even apparent attempts to avoid controversy have backfired. The White House acknowledged it played a role in the arts sales, setting up a legal process to shield the identity of buyers of Hunter Biden pieces, for the stated purpose of ensuring that no one could curry favor with the President by buying his son's art. But critics noted that art sales and anonymity of buyers have long raised concerns about money laundering.

Ukraine business dealings

Biden's involvement in Ukraine has been a major source of his legal and political issues. He served on the board of Burisma, paid as much as $50,000 a month, from 2014 to 2019, according to a Republican-led Senate report on Biden's business activities released in 2020. That partly overlaps with a period during which his father was vice president and tapped by then-President Barack Obama to handle Ukraine issues. The overlap raised concerns about a conflict of interest among some Obama administration officials at the time.

Joe Biden has said his son's work in Ukraine had no influence on his decisions at the time, and State Department critics of Hunter Biden's Ukraine ties say they were never influenced improperly, according to transcripts of Senate testimony.

Federal prosecutors from Justice headquarters in Washington and in Delaware have focused at least in part on whether a lobbying firm working with Burisma called Blue Star Strategies approached US government officials in an attempt to burnish the Ukrainian firm's reputation after State Department officials criticized the oligarch who founded it.

The firm's founders have testified to Congress previously that they were merely interested in understanding the US government's views of the foreign company.

The Justice Department requires lobbyists and public relations firms working on behalf of foreign entities to disclose their ties, and in recent years national security investigators expanded the Department's initiatives to prosecute groups and people who don't publicly disclose their international connections.

Blue Star did not respond to a request for comment.

In the past year, a grand jury in Delaware has issued subpoenas and prosecutors have gathered information about Blue Star Strategies, as well as about Hunter Biden, four people briefed on the matter said. The inquiries included how closely the lobbyists were working with Hunter Biden, how active he was in Blue Star's work on Burisma's reputation in the US, and what their efforts for Burisma entailed, the sources said.

The Blue Star employees previously disclosed in the Senate inquiry that Hunter Biden was included on emails about their work related to Burisma, but said he wasn't particularly involved in the project.

Late last year, witnesses provided interviews in the federal criminal probe. And prosecutors have continued to seek information from some witnesses this year, including about Blue Star Strategies.

Also last year, investigators pursued information from former business contacts of Hunter Biden. His long-time associate and Burisma co-board member Devon Archer went before the Delaware grand jury last July, according to a person familiar with the matter. Archer was asked about the structure of payments Burisma made to an entity founded by Biden and other partners, which appeared to be related Biden's taxes, the source says. He was also asked about lobbying efforts involving Blue Star Strategies, the source said.

Archer was sentenced last month to more than a year in prison for his role in a scheme to defraud a Native American tribe. There hasn't been any contact with investigators since July, the source said.

Archer's attorney, Matthew L. Schwartz of Boies Schiller Flexner LLP, said, "Mr. Archer has cooperated completely with the Delaware U.S. Attorney's Office investigation from the moment he became aware of it."

Archer is in the process of appealing his sentence.

Also among the witnesses to meet with investigators is an Arkansas woman who had a child with Biden and who had sued him for child support, people briefed on the matter said. Her attorney told CNBC that she testified in February to the grand jury in Delaware as part of the investigation and turned over Hunter Biden's financial records she had.

Clint Lancaster, the attorney, didn't respond to CNN requests for comment. Lancaster is not new to the political world: He was listed as an attorney working on a Republican-commissioned review of the 2020 election results, a failed effort to find fraud in the election that President Joe Biden won.

A political live wire

The swirl of partisan politics has hung over the investigation for years. The prospect of the Justice Department continuing to investigate the sitting President's son has added to the sensitivity.

President Joe Biden has said he won't interfere in the independence of the Justice Department. Early on in his presidency, Biden decided to keep in office US Attorney David Weiss, an appointee of former President Donald Trump, to continue to oversee the investigation in Delaware.

As the law enforcement work in Biden's home state has stayed relatively quiet, the political smears of Hunter Biden persisted.

Trump had sought to make the Hunter Biden probe a part of the 2020 campaign, publicly urging Attorney General William Barr to announce an investigation of both Hunter and Joe Biden. Barr responded by publicly announcing that Joe Biden wasn't under investigation.

Giuliani helped orchestrate news stories seeking to tie then-candidate Joe Biden to allegations of corruption in Ukraine. He later helped push stories on documents he said came from a laptop belonging to Hunter Biden.

CNN last year reported that the FBI took possession of the laptop in 2019 and investigators believed it belonged to Hunter Biden.

Hunter Biden, last year making the publicity rounds for his memoir, dodged questions about the laptop.

"There could be a laptop out there that was stolen from me," he said in a CBS interview. "It could be that I was hacked. It could be that it was the -- that it was Russian intelligence. It could be that it was stolen from me."

Giuliani also sought to provide law enforcement a trove of documents he said came from sources in Ukraine and that purported to show wrongdoing by Hunter Biden and his father.

The role of Giuliani and Trump added unusual hurdles for Justice Department officials overseeing the probe, current and former law enforcement officials say.

Barr, concerned about Giuliani's credibility and trying to keep what he viewed as a legitimate investigation of Hunter Biden from being tainted, directed that the documents be handed over to prosecutors in Pittsburgh, according to people briefed on the matter. There, Barr instructed, prosecutors would work with US intelligence agencies to try to determine their authenticity and then turn over any relevant materials to prosecutors in Delaware and elsewhere.

It's unclear whether any of the materials provided by Giuliani or the laptop remain part of the investigation. (CNN)

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