Hunter Biden was indicted Thursday on federal firearms charges, the latest and toughest step yet in a long-running investigation into the president’s son.

Biden is accused of lying about his drug use when he bought a firearm in October 2018, a time during which he has admitted to struggling with an addiction to crack cocaine, according to the indictment filed in federal court in Delaware.

President Joe Biden’s son is also under investigation for his business dealings. The special supervisor on the case has indicated that charges for failing to pay attorney fees on time could be filed in Washington or in California, where he lives.

The indictment comes as Republicans in Congress conduct an impeachment investigation into the Democratic president, largely over Hunter Biden’s business dealings. Republicans have received testimony about how Hunter used the Biden brand to drum up work abroad, but have provided no hard evidence of the president’s wrongdoing.

A gun possession charge against 53-year-old Hunter Biden was previously part of a plea deal that also included guilty pleas on tax charges, but the deal imploded during a July court hearing when a judge raised questions about its unusual provisions.

Defense lawyers have argued that part of the deal, which spares Hunter Biden’s gun count prosecution if he stays out of trouble, remains in place. It contains immunity provisions against other possible charges. Attorneys indicated they would pursue additional charges against him.

However, prosecutors claim the agreement never went into effect and is now invalid.

The gun charge filed Thursday against Hunter Biden is rarely used on its own, and the law could be on shakier legal footing after a recent appeals court ruling found that it does not hold up under the new standards of the Supreme Court for gun laws.

Republicans had dismissed the plea deal as a sweetheart deal. It would have allowed Hunter Biden to serve probation instead of prison time after he pleaded guilty to failing to pay taxes in both 2017 and 2018.

His personal income during those two years totaled about $4 million, including operating and consulting fees from a company he co-founded with the CEO of a Chinese business conglomerate and Ukrainian energy company Burisma, prosecutors said.

Republicans in Congress have continued their own investigation into the Justice Department’s handling of the case and almost every aspect of Hunter Biden’s business dealings, in an effort to tie his financial affairs directly to his father. They have been unable to provide evidence that the president participated directly in his son’s work, although he sometimes dined with his son’s clients or said hello to them during phone calls.