Activity in the investigation had gone covert in recent months due to Justice Department guidelines prohibiting overt actions that could affect an election, the person said.
CNN contacted Biden's attorney and the campaign this week seeking comment about the investigation. On Wednesday, they released a statement acknowledging the probe.
"I learned yesterday for the first time that the U.S. Attorney's Office in Delaware advised my legal counsel, also yesterday, that they are investigating my tax affairs. 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 said in a statement.