WASHINGTON – A whistleblower complaint centering on President Donald Trump's phone call with the Ukrainian president has spurred a number of allegations and counterallegations as Republicans and Democrats jockey for position amid an impeachment inquiry.

At the heart of Congress' probe into the president's actions is his claim that former Vice President and 2020 Democratic frontrunner Joe Biden strong-armed the Ukrainian government to fire its top prosecutor in order to thwart an investigation into a company tied to his son, Hunter Biden.

But sources ranging from former Obama administration officials to an anti-corruption advocate in Ukraine say the official, Viktor Shokin, was ousted for the opposite reason Trump and his allies claim.

It wasn't because Shokin was investigating a natural gas company tied to Biden's son; it was because Shokin wasn't pursuing corruption among the country's politicians, according to a Ukrainian official and four former American officials who specialized in Ukraine and Europe.

Shokin's inaction prompted international calls for his ouster and ultimately resulted in his removal by Ukraine's parliament.

Without pressure from Joe Biden, European diplomats, the International Monetary Fund and other international organizations, Shokin would not have been fired, said Daria Kaleniuk, co-founder and executive director of the Anti Corruption Action Centre in Kiev.

"Civil society organizations in Ukraine were pressing for his resignation," Kaleniuk said, "but no one would have cared if there had not been voices from outside this country calling on him to go."

In a July phone call, Trump asked the president of Ukraine to investigate Biden's actions. That prompted a whistleblower to accuse Trump of asking a foreign government to interfere in the 2020 presidential election, which is now the subject of an impeachment inquiry.

Trump's assertion contradicted

The actions at the center of Trump's allegation occurred in late 2015 and early 2016, when U.S. aid was critical to Ukraine. Russia had seized control of Ukraine's Crimean Peninsula and was supporting separatists who were fighting Ukrainian forces in the eastern part of the country.

Biden took an interest in Ukraine, said Steven Pifer, a William J. Perry fellow at Stanford University and former ambassador to Ukraine under President Bill Clinton.

"You saw the vice president begin to emerge as really sort of the senior policy lead on Ukraine," Pifer said. "It's good to have attention at that level."

At one point, Biden withheld $1 billion in aid to Ukraine to pressure the government to remove Shokin from the Prosecutor General's Office.

Trump and his personal attorney Rudy Giuliani claim Biden did this to quash Shokin's investigation into Ukraine's largest gas company, Burisma Holdings, and its owner, oligarch Mykola Zlochevsky.

They say this benefited Biden's son, Hunter Biden, who served on Burisma's board of directors – for which he was paid $50,000 a month.

Their assertion is contradicted by former diplomatic officials who were following the issue at the time.

Burisma Holdings was not under scrutiny at the time Joe Biden called for Shokin's ouster, according to the National Anti-Corruption Bureau of Ukraine, an independent agency set up in 2014 that has worked closely with the FBI.

Shokin's office had investigated Burisma, but the probe focused on a period before Hunter Biden joined the company, according to the anti-corruption bureau.

The investigation dealt with the Ministry of Ecology, which allegedly granted special permits to Burisma between 2010 and 2012, the agency said. Hunter Biden did not join the company until 2014.

Read it yourself: The full declassified text of the Trump whistleblower complaint

Critics of Hunter Biden have questioned how he landed such a lucrative role with no experience in Ukraine or the gas industry.

But it's not unusual for Ukrainian companies to bring on high-profile people from the West in an effort to burnish their image and gain influence, Pifer said.

Cofer Black, who served as Bush's CIA counterterrorism chief, joined Burisma's board in 2017.

There is no evidence Hunter Biden did anything wrong, said Yuri Lutsenko, the prosecutor general who succeeded Shokin.

However, Lutsenko, who's also faced criticism for his actions as prosecutor, supported Trump's claim before changing his story. He resigned as prosecutor in August.

The Burisma investigation ended with a settlement and a fine paid by one of the firm's accountants, according to Sergii Leshchenko, a former Ukrainian lawmaker who spearheaded anti-corruption efforts under former President Petro Poroshenko.

How Biden leveraged U.S. aid to oust prosecutor

In the wake of the 2014 ouster of pro-Russian president Viktor Yanukovych, European and U.S. officials stepped up their efforts to deal with corruption in Ukraine.

"A big part of our diplomacy was pushing the Ukrainian government to clean up the corruption, partly because it was that corruption that allowed Russia to manipulate the country politically and economically," said Charlie Kupchan, who served as a special assistant to President Barack Obama and a senior director for European Affairs on the National Security Council.

Biden used U.S. aid as "a stick to move Ukraine forward," Kupchan said. "He was acting alongside our European allies. Everybody was of a single mind that this prosecutor was not the right guy for the job."

Biden has boasted about his role in getting Shokin fired. During a 2018 speech at the Council on Foreign Relations, he said he withheld $1 billion in loan guarantees for Ukraine in order to force the government to address the problem with its top prosecutor.

"I looked at them and said: 'I’m leaving in six hours. If the prosecutor is not fired, you’re not getting the money. Well, son of a bitch. He got fired. And they put in place someone who was solid at the time," he said.

Pifer, who also oversaw diplomacy with Russia and Ukraine under President George W. Bush, said it was appropriate for Biden to use U.S. aid as leverage. He said he used similar methods to pressure Ukraine.

Even without any credible evidence that Joe Biden sought to benefit his son, Pifer said Hunter Biden showed poor judgment in getting involved with Ukraine.

"At that point, it was pretty clear that his father was already the lead American policy person on Ukraine," he said. "And even if there's no conflict of interest, I think that he should have been more mindful of how that appears."

International effort to fight corruption in Ukraine

The international effort to remove Shokin, who became prosecutor general in February 2015, began months before Biden stepped into the spotlight, said Mike Carpenter, who served as a foreign policy adviser to Biden and a deputy assistant secretary of defense, with a focus on Ukraine, Russia, Eurasia, the Balkans, and conventional arms control.

As European and U.S. officials pressed Ukraine to clean up Ukraine's corruption, they focused on Shokin's leadership of the Prosecutor General's Office.

"Shokin played the role of protecting the vested interest in the Ukrainian system," said Carpenter, who traveled with Biden to Ukraine in 2015. "He never went after any corrupt individuals at all, never prosecuted any high-profile cases of corruption."

That demonstrated that Poroshenko's administration was not sincere about tackling corruption and building strong, independent law enforcement agencies, said Heather Conley, director of the Europe program at the Center for Strategic and International Studies, a Washington-based foreign policy think tank.

In July 2015, Shokin's office became mired in scandal after authorities raided homes belonging to two high-ranking prosecutors. Police seized millions of dollars worth of diamonds and cash, suggesting the pair had been taking bribes.

It became known as the "diamond prosecutors" case. Deputy General Prosecutor Vitaliy Kasko, who said he tried to investigate it, resigned months later, calling the prosecutor's office a "hotbed of corruption" and an "instrument of political pressure."

Shokin's office also stepped in to help Zlochevsky, the head of Burisma.

British authorities had frozen $23 million in a money-laundering probe, but Shokin's office failed to send documents British authorities needed to prosecute Zlochevsky. The case eventually unraveled and the assets were unfrozen.

In October 2015, Ukrainians staged a protest outside Poroshenko's home calling for Shokin's removal.

Pressure mounts to remove Shokin

In late 2015, U.S. officials stepped up the pressure.

During a September 2015 speech at a financial forum in Odessa, U.S. Ambassador to Ukraine Geoffrey Pyatt decried the inability of Shokin's office to root out corruption.

Biden and Ukraine: Will Trump's efforts to discredit the former VP hurt him?

"Rather than supporting Ukraine's reforms and working to root out corruption," Pyatt said, "corrupt actors within the Prosecutor General’s Office are making things worse by openly and aggressively undermining reform."

In October 2015, then-Assistant Secretary of State Victoria Nuland told the Senate Foreign Relations committee the Prosecutor General's Office must lock up "dirty personnel" in its own office.

In December 2015, Biden railed against the "cancer of corruption" in a speech before the country's parliament and called out Shokin's office.

Besides Biden's threat over the $1 billion in aid, the International Monetary Fund threatened to delay $40 billion in aid for similar reasons.

Shokin was eventually removed from his position in the spring of 2016.

The decision to remove Shokin "creates an opportunity to make a fresh start in the Prosecutor General's Office," said Jan Tombinski, the EU's ambassador to Ukraine, in a written statement.

"I hope," Tombinski said, "that the new Prosecutor General will ensure that the Office of the Prosecutor General becomes independent from political influence and pressure and enjoys public trust."

Contributing: Kim Hjelmgaard