At a news conference in Kyiv on Friday, Ukraine's newly appointed top prosecutor announced a sweeping review of past corruption investigations that had been either shut down or split up. Fifteen of those cases, according to an official press release, involve the founder of the Ukrainian gas firm Burisma.

Former Vice President Joe Biden's son Hunter was appointed to Burisma's board in 2014, while his father was leading policy on Ukraine during the Obama administration.

The audit of earlier corruption probes follows a promise Ukrainian President Volodymyr Zelenskiy made to President Trump in a July 25 phone conversation: that a new prosecutor general would look into the closing of an investigation into Burisma's practices.

Ukraine might appear to be bowing to pressure from Trump, who lifted his previously unannounced two-month hold on nearly $400 million in security assistance for Ukraine on Sept. 11. Earlier this week, the U.S. State Department gave the green light for Congress to consider selling 150 Javelin anti-tank missiles worth nearly $40 million to Ukraine. Zelenskiy had mentioned his desire to acquire those weapons, which are intended to counter Russian-backed separatists in eastern Ukraine, in his phone call with Trump.

Still, it's not clear the official review of dropped cases involving Burisma owner Mykola Zlochevsky would include the gas firm he founded. Those cases date to his time as ecology minister for Viktor Yanukovich, the Ukrainian president whose 2014 overthrow took place before Hunter Biden was hired to Burisma's board. Biden left that board earlier this year.

Former New York Mayor Rudy Giuliani, who is Trump's personal lawyer, has repeatedly accused Joe Biden of pushing in 2016 for the removal of Ukraine's then-top prosecutor, Viktor Shokin, to prevent him from investigating Burisma.

Biden responded to those charges Wednesday in Reno, Nev. "What I did in Ukraine was carry out the official policy of the United States government: to root out corruption in Ukraine in conjunction with our European allies, the International Monetary Fund and its leader, our closest democratic allies," he said. "It was a fully transparent policy, carried out in front of the whole world, and fully, fully embraced by the international community of democracies."

"The key word [in this audit] is neither Biden nor Burisma," Prosecutor General Ruslan Ryaboshapka, whom Zelenskiy appointed in late August, told reporters. "The key word is those cases that were either closed or investigated under the previous management, and among them, there may be those with those two words."

Asked if he had found any evidence of wrongdoing by Hunter Biden, Ryaboshapka replied, "I have no such information."

The review of previously closed corruption probes is required by legislation that also terminates the investigative powers of the prosecutor general's office after Nov. 20, 2019.

The announcement of the audit came hours after a trove of Ukraine-related text messages exchanged by U.S. diplomats was released Thursday evening by House Democrats who have begun an inquiry into the impeachment of President Trump.

One of those messages came from the top U.S. diplomat in Kyiv, chargé d'affaires Bill Taylor. "I think it's crazy to withhold security assistance for help with a political campaign," Taylor wrote to the U.S. ambassador to the European Union, Gordon Sondland.

After conferring with Trump, Sondland texted a reply to Taylor. "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear no quid pro quo's of any kind."

Sondland, a hotel developer who was named ambassador after donating $1 million to Trump's inauguration festivities, then suggested they quit conversing by text messages.

Both Sondland and Taylor are expected to be called to testify in the House impeachment inquiry.

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