The broad outlines of how the Bidens’ roles intersected in Ukraine have been known for some time. The former vice president’s campaign said that he had always acted to carry out United States policy without regard to any activities of his son, that he had never discussed the matter with Hunter Biden and that he learned of his son’s role with the Ukrainian energy company from news reports.

But new details about Hunter Biden’s involvement, and a decision this year by the current Ukrainian prosecutor general to reverse himself and reopen an investigation into Burisma, have pushed the issue back into the spotlight just as the senior Mr. Biden is beginning his 2020 presidential campaign.

They show how Hunter Biden and his American business partners were part of a broad effort by Burisma to bring in well-connected Democrats during a period when the company was facing investigations backed not just by domestic Ukrainian forces but by officials in the Obama administration. Hunter Biden’s work for Burisma prompted concerns among State Department officials at the time that the connection could complicate Vice President Biden’s diplomacy in Ukraine, former officials said.

“I have had no role whatsoever in relation to any investigation of Burisma, or any of its officers,” Hunter Biden said Wednesday in a statement. “I explicitly limited my role to focus on corporate governance best practices to facilitate Burisma’s desire to expand globally.”