Ukrainian prosecutors say three separate corruption investigations have been stymied because American law enforcement officials won’t turn over evidence from a key witness, Paul Manafort, the president’s former campaign chair.

Special investigators within the Prosecutor General’s Office are investigating nearly a decade of work by Manafort on behalf of former Ukrainian president Viktor Yanukovych, a close ally of Vladimir Putin, and his political group, the Party of Regions.

But Manafort, 68, also faces criminal charges in the United States, and the FBI and US Department of Justice have been reluctant to share testimony, documents, and other evidence they have collected. Manafort was indicted in the US in October and accused of conspiracy to commit money laundering by the Office of the Special Counsel, led by Robert Mueller, which is examining Russian interference in the 2016 election and possible collusion with President Donald Trump’s campaign.

Manafort’s lawyers have denied these charges in court. A spokesperson for Manafort, Jason Maloni, declined to comment for this story, citing a gag order issued by the federal judge. Manafort has said in the past his work in Ukraine was legitimate and above board.

In Ukraine, authorities are eager to talk to Manafort. Serhiy Gorbatyuk, head of the special investigations department, called Manafort a “key witness” in three ongoing criminal probes: suspected money laundering by his former lobbying firm, Davis Manafort; a lucrative contract he steered toward the powerful American law firm Skadden, Arps, Slate, Meagher & Flom; and an investigation of the so-called black ledger, a document that reportedly shows illicit payments by the past regime.

Prosecutors in Kiev said they have made seven separate appeals over the past two years for help in questioning President Donald Trump's former campaign manager, including letters to then-FBI director James Comey and US Justice Department officials. The two countries have an agreement to cooperate during criminal investigations, but Ukraine has so far not received all the help it needs, prosecutors there say, leaving the inquiries in danger of fizzling out.

Gorbatyuk told BuzzFeed News that his last interaction with the Justice Department was in September. He said an FBI agent agreed to question Manafort and employees of Skadden, Arps, and that the Justice Department said it would turn over documents.

Gorbatyuk said the Justice Department has released some information, such as emails and an important contract, but has not released interviews with Manafort and employees of Skadden, Arps.

“We haven’t received an official response,” said Andriy Radionov, another prosecutor in Gorbatyuk's unit.

The FBI and the Justice Department declined to comment.

In one of the three investigations by Ukrainian authorities, prosecutors are examining Ministry of Justice officials for a 2012 contract with Skadden, Arps, the US law firm. Ukraine hired the firm to conduct a review of what happened to former prime minister Yulia Tymoshenko, who was jailed after she challenged Yanukovych during the 2010 presidential election.

Tymoshenko's supporters say her imprisonment is politically motivated. When the Ministry of Justice sought an independent third party to examine the case, Skadden, Arps was awarded a $1.1 million contract without competition, prosecutors said.

But Manafort reportedly boosted that amount by secretly sending an additional $4 million to the law firm through an offshore account he controlled with his partner, Rick Gates, according to the US indictment by the Special Counsel’s Office. The US State Department criticized the report by Skadden, Arps, saying the firm went soft on Manafort’s former boss, Yanukovych.

“Skadden, Arps lawyers were obviously not going to find political motivation if they were not looking for it,” said Victoria Nuland, a spokesperson for the State Department. The report was “incomplete and doesn’t give an accurate picture,” Nuland said in 2012.

Skadden, Arps, Slate, Meagher & Flom did not return messages seeking comment. In the past, a spokesperson has said that the firm would cooperate with authorities.

Three years after the General Prosecutor’s Office began investigating, the law firm refunded to the Ukrainian government $567,000 that the firm said had been in an escrow account. Ukrainian prosecutors investigating the matter said that once the money was returned, a high-ranking official at their department, Yevgeny Enin, stopped pushing the US to turn over records and interviews. This has dampened their ability to further understand what happened.

Enin declined to comment.

In the second investigation, a Ukrainian member of parliament, Serhiy Leshchenko, has accused Manafort’s company of money laundering and provided to investigators an invoice reportedly recovered from a safe in Manafort’s former Kiev office. The document shows a consulting company, Davis Manafort, billed an offshore entity based in Belize $750,000 for the sale of 501 computers in 2009. A former principal at the company did not return messages for comment, but has said in the past that the work in Ukraine was proper.

“The original copy was confiscated by the FBI in February,” Leshchenko told BuzzFeed News of the Belize transaction.

The final probe involves the "black ledger," which Ukrainian investigators have been investigating as evidence of off-the-books payments from Yanukovych’s Party of Regions.

In it, prosecutors say there is evidence of a campaign to clean up the foreign reputation of former president Yanukovych. According to this document, Manafort received $12.7 million for his work on the Party of Regions. A $750,000 entry is listed next to Manafort’s name on the same day his company sold the computers to Belize.

Gorbatyuk said the transaction is suspicious and is being investigated for money laundering.

Manafort’s signature doesn't appear in the ledger. But Vitaly Kalyuzhny, a former Ukrainian member of parliament, signed nine times next to payments to Manafort — including the $750,000 computer payments.

Kalyuzhny was the founding member of the European Centre for a Modern Ukraine, a nongovernment organization based in Brussels that lobbied in favor of Yanukovych.

The organization was identified in the indictment against Manafort and Gates as having contracted with American lobbying companies. Kalyuzhny is now hiding from Ukrainian law enforcement and could not be reached for comment. Prosecutors said they believe he is in Russia.

In addition to trouble obtaining evidence from US authorities, the three Ukrainian cases face another problem: a turf war between Ukrainian law enforcement officials.

In November, the top prosecutor in Ukraine ruled that the investigation must be transferred to a wholly new department, the National Anti-Corruption Bureau of Ukraine. Known as NABU, the bureau was founded in 2014 as part of a wave of measures imposed on Ukraine by the International Monetary Fund and the European Union to root out graft and kleptocracy.

But NABU told BuzzFeed News it has not received the cases — leading to a logjam where officials in both agencies said they no longer have access to sensitive cases' files.

"This is an open obstacle to the investigation,” Gorbatyuk said.