Washington — As a special assistant to President Trump, Kash Patel's area of responsibility includes counterterrorism, but over the last few weeks, he's also been enmeshed in Ukraine. One witness suggested he may have been part of a back channel to the president on Ukraine, and the 300-page impeachment report released by House Intelligence Committee Democrats Tuesday said that Patel spoke with Rudy Giuliani, the president's personal attorney, in the spring, before nearly $400 million in military aid to Ukraine was suspended.

According to the call records revealed in the report, Patel had a 25-minute phone conversation with Giuliani on May 10. Five minutes after their call, Giuliani spoke with an unidentified number for 17 minutes and then with associate Lev Parnas, a Ukrainian-American who has been accused of illegally funneling foreign money to U.S. political candidates and of aiding Giuliani in his Ukraine investigations.

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In an exclusive interview with CBS News senior investigative correspondent Catherine Herridge, Patel denied the Giuliani call touched on Ukraine.

"That was a personal conversation. I was delighted to have with the mayor of New York, where I grew up," he told Herridge Wednesday. Patel then challenged the Democrats' report.

"If they had any actual information as to the substance of those calls, then they would've put it in there," he said. "But they didn't. This is a standard operating procedure to castigate someone's name and reputation — especially when they work in the White House."

The impeachment report contained only the dates, times and the duration of the calls, not the content.

Patel said he was "never a back channel to President Trump on Ukraine matters, at all, ever. Never — no meetings, no shuttling of documents, no meetings in secret. Never happened. I have no idea where they got that from."

And Patel, asked whether he has ever discussed Ukraine with the president, categorically denied this was the case.

"Absolutely not. Not ever," he said.

Patel says he is mystified by his involvement in the impeachment inquiry, though he said he suspects it may have something to do with his prior employment. Before he worked at the White House, Patel was a top aide to Congressman Devin Nunes on the House Intelligence Committee, and in that capacity, he authored a memo questioning the FBI and Justice Department's actions to obtain a surveillance warrant for a former Trump campaign aide.

"It's the same committee — the best I can come up with — the same folks on the Intelligence Committee now are the same folks that I worked with, and it became such a partisan political environment. And I think it has increased, so any shot they can take at the president they take, and as you've noted, they have repeatedly have taken them at me.

Fiona Hill in her closed-door testimony before Congress in October said that she believed, based on a conversation with an aide to the president, that Mr. Trump thought that Patel was the Ukraine director for the National Security Council and that he and Patel had exchanged some material on Ukraine. She raised her concerns with then-deputy national security adviser Charles Kupperman, removed Patel from internal distribution lists and warned her office "to be very careful" about communication with Patel until it became clear what Ukraine materials he may have been sending to the president.

Hill also said in her testimony that she has never had a conversation with Patel, though she knew who he was from meetings they had both attended.

A House Intelligence Committee spokesperson told CBS News, "the Committee welcomes any and all documents from the White House, including any that would support Mr. Patel's claim that he only happened to speak with Giuliani because he had been the mayor of the city where Patel grew up, and their lengthy conversation didn't involve any of the Ukraine work Giuliani was doing for the President."

To the contrary, the spokesperson continued, Hill "testified she had been informed that Patel was providing information to the President on Ukraine as part of an alternate channel, and this call took place right around that time. If the White House had exculpatory documents of any kind, they would have turned them over instead of refusing to comply with our lawful and duly authorized subpoenas."

Herridge also asked Patel about congressional Republican claims that Ukraine — not Russia — interfered in the 2016 presidential election.

"That," he said, "is not something I worked on at House Intel. We ran a Russian active measures investigation into the 2016 election. Whether there was a Ukrainian involvement or not, I am not able to comment."

Patel has taken the unusual step of filing lawsuits against media organizations that publicized Ukraine-related allegations against him. His former boss, Nunes, also filed a lawsuit this week against CNN, and he is also suing several media outlets. Both men are using the same lawyer.

"It's time that I began fighting back," he said. "I filed lawsuits for $75 million. And I'll file $75 million more. If they wanna attack my character for things that never occurred, I'm happy to do that."