Hillary Clinton’s campaign is questioning Donald Trump’s top political aide’s ties to a pro-Kremlin political party in the Ukraine and claiming it is further evidence of the Republican nominee’s cozy relationship with Russia.

The New York Times reported that handwritten ledgers found in the Ukraine show $12.7 million in undisclosed payments to Paul Manafort, Trump’s campaign manager, from the pro-Russia party founded by the country’s former president Viktor Yanukovych. Investigators are probing whether the money was part of an illegal off-the-book system that may have also made payments to election officials, according to the Times in a story published Sunday night.

“Given the pro-Putin policy stances adopted by Donald Trump and the recent Russian government hacking and disclosure of Democratic Party records,” Clinton campaign manager Robby Mook said in statement, “Donald Trumphas a responsibility to disclose campaign chair Paul Manafort’s and all other campaign employees’ and advisers’ ties to Russian or pro-Kremlin entities, including whether any of Trump’s employees or advisers are currently representing and or being paid by them.”

Manafort, who has had several international clients including Philippine dictator Ferdinand Marcos, denied any wrongdoing.

“The simplest answer is the truth: I am a campaign professional,” said Manafort in a statement released Monday. “I have never received a single ‘off-the-books cash payment’ as falsely reported by The New York Times, nor have I ever done work for the governments of Ukraine or Russia.”

He added that his five-year stint as doing work as a political consultant in the Ukraine ended after that country’s elections in 2014.

Trump’s relationship with Russia has drawn scrutiny throughout the campaign, most recently when he encouraged hackers from that country to find Clinton’s missing emails, an apparent invitation for a foreign power to intervene in a U.S. election. Trump also has frequently praised Russian President Vladimir Putin.