Manafort was indicted last month in Washington on charges he denies ranging from money laundering to acting as unregistered agent of former pro-Kremlin president Viktor Yanukovich’s Party of Regions. The charges, some going back more than a decade, mention neither Trump nor his campaign, Reuters reports

Serhii Horbatiuk, head of special investigations at the general prosecutor’s office, told Reuters Ukraine had sent requests in 2014 and 2015 to question representatives of a law firm and Manafort.

Read alsoBloomberg: Manafort charges renew questions about Skadden's work in UkraineHe said Ukraine had received reassurances from the Federal Bureau of Investigation (FBI) that the requests would be met, but without result.

“Of course it’s not normal,” Horbatiuk said in an interview at his office.

“I would not say that we are upset, I would not say officially that we are upset, but it’s not clear why it is so,” he said. “Even the results they have got so far, we could have got them back in 2015 or 2016 with their help. But during these years there was no cooperation for unknown reasons.”

Read alsoDonald Trump Jr. communicated with WikiLeaks during 2016 campaignIn Washington, the FBI and the Department did not immediately respond to requests from Reuters for comment.

Horbatiuk is investigating two cases.

One relates to the suspected illegal use of Ukrainian state funds by justice ministry officials to pay a U.S. law firm for a report used to justify the imprisonment of former prime minister and political rival Yulia Tymoshenko during Yanukovich’s rule.

The second relates to a so-called ‘Black Ledger’ discovered after Yanukovich’s 2014 ouster by street protests, a book supposedly listing payments from a slush fund by Yanukovich’s Party of Regions to their associates, including Manafort.

Yanukovich denies corruption accusations.

Manafort denies all wrongdoing and in Ukraine he is at this stage only wanted as a witness.

Horbatiuk said he had not been contacted for assistance by the team of Special Council Robert Mueller, who indicted Manafort during an investigation into alleged Russian efforts to swing the 2016 presidential election in Trump’s favor.

Manafort’s attorney says his work for the Ukrainians ended in 2014, two years before he joined the Trump campaign.

But the indictment sheds new light on the Ukrainian cases: the allegation that Manafort used offshore accounts to funnel $4 million to pay for the Tymoshenko report was news to Horbatiuk’s team, who thought the report had cost around $1 million.

He will submit a new request for help to the U.S. within weeks.

Read alsoNewsweek: Pro-Putin party in Ukraine gave Paul Manafort $600,000 a month“It is important to combine our investigations so that we can obtain information to determine the origin of this money ($4 million),” he said.

“And the general information allows us to say that the investigation could be promising provided that the law enforcement agencies of the United States and Ukraine cooperate in what we are interested in.”

Manafort joined the Trump presidential campaign in March 2016 and later became campaign manager, but he was forced to resign in August as questions emerged about his previous work for Yanukovich’s party.