(NewsNation) ⁠— The 2018 federal tax investigation into President Joe Biden’s son, Hunter Biden, is intensifying as prosecutors dig into his financial dealings abroad, and they have since expanded their focus to include potential violations of foreign lobbying and money-laundering rules. His business dealings in Ukraine were at the center of the controversy that led to former President Donald Trump’s first impeachment.

Tune in to NewsNation at 9/8 Central to watch an hourlong “Dan Abrams Live” special that takes an unbiased and nonpartisan deep look at the Hunter Biden investigation, including background from two former federal prosecutors who have followed the case.

The allegations and the resulting investigation are complex and multifaceted. So, here’s a look at the probe so far:

From 2014 to 2019, Hunter Biden served on the board of Ukraine-based Burisma and was paid about $50,000 a month. That prompted Trump to urge Ukrainian President Volodymyr Zelenskyy to open an investigation into Biden’s father, who was Trump’s electoral opponent at that time. Hunter Biden’s involvement with Burisma also sparked concerns about a potential conflict of interest given that elder Biden was deeply involved in U.S. foreign policy with Ukraine when he was vice president in President Barrack Obama’s administration.

Then in May 2020, Senate Republicans issued a subpoena seeking documents from the younger Biden and asked for information related to more than two dozen entities, including Burisma. The subpoena specifically sought records from Blue Star Strategies from Jan. 1, 2013 to the present “related to work for or on behalf of Burisma Holdings or individuals associated with Burisma,” The Hill reported at the time. His work in Ukraine was at the center of the controversy that led to Trump’s first impeachment.

Hunter Biden went on to confirm the investigation’s existence in December 2020, one month after the presidential election. He said in a statement at the time that he was “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.”

He also said in an April 2021 interview with CBS that he wasn’t sure if a laptop — one he allegedly left at a Delaware repair shop and that reportedly had information about his business dealings in Ukraine — was his. He told CBS reporter Tracy Smith that the laptop could have been his or it “could be I was hacked. It could be that it was Russian intelligence.”

The laptop surfaced publicly in October 2020, when The New York Post reported on emails that it said had come from Hunter Biden’s laptop and that it said were received from Rudy Giuliani, who was Trump’s personal lawyer at the time. A third person familiar with the matter said in 2020 the tax investigation does not have anything to do with the laptop.

More recently, the Wall Street Journal published a report that said prosecutors from the U.S. attorney’s office in Delaware sought more information related to the income Hunter Biden received from Burisma Holdings Ltd. Prosecutors also questioned one of Hunter Biden’s associates about his drug and alcohol abuse, spending habits and mental state in 2018, WSJ reported.

The New York Times additionally reported in March that Hunter Biden paid off a significant tax liability. The newspaper noted, however, that the investigation is ongoing, citing “people familiar with the case.”

As the investigation continues, ABC News has released a report claiming that the Secret Service allegedly spends more than $30,000 each month renting out a Malibu mansion in California to protect Hunter Biden.