Ukrainian prosecutor reviewing cases involving company that employed Joe Biden's son

KYIV, Ukraine -- Ukraine's prosecutor general said at a press conference his office is reviewing past criminal cases involving the businessman whose energy company employed Hunter Biden on its board.

Ruslan Ryaboshapka said none of the cases under review involve the former vice president's son. The focus of the investigation is on two businessmen, including Mykola Zlochevsky, owner of the energy company Burisma.

The review comes amid the impeachment inquiry into President Donald Trump's alleged attempts to pressure Ukraine's government into investigating the Bidens, and the timing raises questions whether Ukraine's new leadership was perhaps making a concession to Trump's demands. In a July 25 phone call, Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Biden and his son, triggering the impeachment inquiry from House Democrats into whether the U.S. president had abused his office for personal political gain.

Speaking Friday in Kyiv, Ryaboshapka said his office was reviewing at least 15 cases involving Zlochevsky and Sergey Kuchenko, who founded another Ukraine energy firm.

Zlochevsky, a billionaire oligarch and the former minister of natural resources, previously had faced multiple investigations into possible fraud or corruption. For years he has been accused of financial crimes by anti-corruption activists. He was a close associate of Viktor Yanukovych, Ukraine's former president who was toppled by mass protests in 2014.

After the revolution, Hunter Biden was appointed to Burisma's board. That move was criticized as unethical by anti-corruption activists, who suggested the younger Biden had been appointed for his name and potential family connections at a moment when Zlochevsky was under threat of investigation.

Trump and his allies have sought to advance an unsupported theory that Vice President Joe Biden used his position to shield his son and Burisma from investigation by pushing Ukraine's government to fire its then-prosecutor general, Viktor Shokin.

No evidence has emerged to support that theory. In reality, most major Western governments, as well as international financial institutions and anti-corruption activists, also had demanded Shokin's removal. Many viewed him as a block on reform and criticized him for failing to investigate several prominent corruption allegations. Shokin's failed investigation into Burisma was cited as one of the reasons for his removal.

Former senior Ukrainian law enforcement officials have told ABC News they believe the theory about Biden was developed by Ukraine's most recent prosecutor general, Yuri Lutsenko, in an attempt to save his position before Zelenskiy fired him over the summer.

Nonetheless, Trump has continued to demand Ukraine investigate Biden over alleged wrongdoing tied to Burisma, with the effort spearheaded by his personal attorney, Rudy Giuliani.

Text messages emerged this week showing two senior State Department officials had been working with Giuliani to press Zelenskiy to make a public statement promising to investigate the Bidens, as well as alleged interference by Ukraine in the U.S. 2016 election.

The messages released by a Democratic congressman on Thursday showed Trump's special envoy to Ukraine, Kurt Volker, and his ambassador to the European Union, Gordon Sondland, advised a key Zelenskiy aide that the Ukrainian president had to make the pledge before Trump would agree to meet with him.

In the messages, the aide, Andriy Yermak, appeared to agree, writing that Zelenskiy was ready to make a statement, but negotiations broke down after Yermak said the Ukrainians needed to agree on a date for Zelenskiy to travel to the U.S. before doing so.

Zelenskiy told ABC News he played no role in Ryaboshapka's decision to review the 15 cases.

"Not involved at all," said Zelenskiy, walking past a scrum of cameras ahead of a panel with Belarus President Alexander Lukashenko in the town of Zhytomyr.

Ryaboshapka, who was appointed by Zelenskiy, denied he had faced any pressure to start the reviews or that he had spoken to any foreign representatives about it, including Giuliani.

Anti-corruption activists have long said some of the investigations into Burisma and Zlochevsky, cases tied to tax fraud or abuse of power, were allowed to collapse under a series of former prosecutors, including Shokin and Lutsenko.

Ukraine's prosecutor's office said in a statement if any illegal activity is found in the cases under review, they'll take action.

"We only recently started to work, only recently started to audit the cases," Ryaboshapka said.