(The Hill) – A federal tax investigation into President Joe Biden’s son Hunter Biden is ramping up as prosecutors gather more information about his financial dealings abroad, according to a report by The Wall Street Journal.

Prosecutors from the U.S. attorney’s office in Delaware sought more information related to the income Biden received from Ukrainian natural gas company Burisma Holdings Ltd., a source told The Wall Street Journal. They also questioned one of his associates about his drug and alcohol abuse, spending habits and mental state in 2018. 

An investigation into Biden for tax crimes began in 2018 and expanded to include potential violations of foreign lobbying and money laundering rules. His work in Ukraine contributed to former President Donald Trump’s first impeachment. From 2014 to 2019, Hunter Biden served on Burisma’s board and was paid about $50,000 a month. That prompted Trump to urge Ukrainian President Volodymr Zelenskyy to open an investigation into Biden’s father, who was Trump’s electoral opponent at that time.

Prosecutors are also reportedly investigating Hunter Biden’s business relationships in other countries, including China and Kazakhstan. 

Hunter Biden said earlier this month that he paid off more than $1 million in tax liability with a loan, according to The New York Times. However, that doesn’t safeguard him from charges. 

The Journal reported that it did not know how close to complete the investigation was.