(The Hill) – A federal tax investigation into 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 Trump’s first impeachment. Between 2014 and 2019, Hunter Biden served on Burisma’s board and was paid around $50,000 a month, which prompted Trump to urge President Volodymr Zelensky to open up an investigation into Biden’s father, President Biden, who was Trump’s electoral opponent at that time.

Prosecutors are also reportedly investigating Hunter Biden’s business relationships in other countries, such as 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 the investigation was from being complete.