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(CNN) —  

Phone records released this week as part of the House Democrats’ impeachment report revealed a series of calls from President Donald Trump’s personal attorney, Rudy Giuliani, to an “unidentified” number designated simply as “-1” that has investigators wondering who it is.

The phone company logs, according to the report, also show Giuliani made several calls to numbers associated with the White House and the Office of Management and Budget during that same time frame, further fueling speculation about who he was talking to and for what purpose.

While the substance of these conversations remains unclear, phone records reveal a flurry of calls involving Giuliani in April that could be relevant in the ongoing impeachment inquiry into Trump’s dealings with Ukraine, according to the report by Democrats on the House Intelligence Committee.

The calls also came at a time when Giuliani was helping deal with the fallout from special counsel Robert Mueller’s report on Russian interference in the 2016 presidential election after a redacted version was released that same month, one White House official noted Wednesday.

Now, months after they took place, those calls are at the center of a mystery that could have significant implications on the impeachment process.

House Intelligence Committee chairman Adam Schiff was quick to imply that Trump himself is connected to the “-1” listing, despite telling CNN’s Anderson Cooper that his panel is still investigating the identity of the individual.

“Rudy Giuliani only had one client – and that was Donald Trump,” the California Democrat said.

But several members on the House Intelligence Committee told CNN that they did not know whether the numbers were traced to various individuals or who “-1” was.

Trump’s acting chief of staff and OMB director, Mick Mulvaney, immediately emerged as a possible source of the OMB calls due to his role in helping facilitate the hold on aid to Ukraine and influence at the agency despite transitioning to the White House last December.

A spokesperson for OMB told CNN that no one from the agency was ever in contact with Giuliani, despite what’s shown on the logs in the report.

A White House official told CNN that Mulvaney and Giuliani have not discussed Ukraine and Mulvaney’s call records show he did not speak to the President’s personal attorney by phone on the dates listed in the report.

Giuliani also insists that he was not involved in withholding the aid to Ukraine and that he never discussed the issue on any call to OMB.

“(I) don’t remember calling OMB and not about military aid never knew anything about it,” he told CNN via text on Wednesday.

Concerns about Giuliani

Amid all the uncertainty related to these calls, the release of the records has already raised additional concerns about Giuliani.

Some close to the President and his campaign believe Giuliani has become a “liability” for the President, two sources told CNN on Tuesday.

A Trump campaign source reacted to the findings from House Intelligence Committee, including the calls with White House and OMB officials, with a finger pointed squarely at Giuliani, saying: “Rudy should be worried.”

However, Trump has not publicly wavered in his support for Giuliani, and despite the lingering questions, he is still part of the President’s legal team.

Still, the call records involving Giuliani and “-1” are of particular interest.

House Democrats who have cast the call records as a paper trail detailing Giuliani’s coordination with top administration officials, and potentially even the President himself, in his dealings with Ukraine that are central to the impeachment probe.

The committee’s impeachment inquiry report shows Giuliani speaking a number of times with that individual during several key moments detailed in the document.

House investigators obtained the records from Verizon and AT&T, which is the parent company of CNN, but the logs included in the House report only provide details related to the length of the call and its participants.

“We can’t confirm yet who the number belongs to,” Schiff told CNN Tuesday.

“We are certainly investigating that,” he added, noting that the phone number “-1” also came up during the trial of former Trump adviser Roger Stone.

Prosecutors showed evidence at Stone’s trial that he took a call coming from a “-1”number in August 2016, and a government witness’ testimony implied it could have come from Trump.

That call occurred during the 2016 presidential campaign before Trump entered the White House and witnesses offered no explanation as to where the “-1” designation came from or if it was unique to Trump himself.

A source told CNN that the “-1” listing may only show up on call logs, not on caller ID from the White House, meaning the recipient could see “202” or “unknown” on caller ID but the call could show up as “-1” on phone company call logs.

CNN’s Pamela Brown, Dana Bash, Lauren Fox, Rene Marsh, Ellie Kaufman and Sara Murray contributed to this report.