House Intelligence Chairman Adam Schiff (D-California) said Tuesday night that his committee is investigating the identity of the owner of the mystery phone number “-1” that appeared in the Intelligence Committee’s 300-page report on its findings from the impeachment hearings into President Donald Trump.

Phone records obtained from Verizon and AT&T revealed that Trump attorney Rudy Giuliani had numerous calls with a phone number listed only as “-1” in April. Schiff has hinted that he believes “-1” could be the President himself.

“We can’t confirm yet who” the number belongs to,” Schiff said during an interview on CNN. “We are certainly investigating that. Rudy Giuliani only had one client, and that was Donald Trump.”

The phone records in the Intelligence Report revealed communications in April between Giuliani and Ranking Member of the Intelligence Committee Devin Nunes (R-California) and between Nunes and Lev Parnas, Giuliani’s associate who was heavily involved in the Ukraine pressure campaign.

The records show Parnas also had phone conversations with Giuliani, with conservative media executive John Solomon and with conservative lawyer Victoria Toensing.

Giuliani’s records indicate he had also been in contact with Toensing, as well as with fellow Trump lawyer Jay Sekulow. Giuliani is only person listed in the report as being in contact with “-1.”

In the interview, Schiff mentioned that the phone number -1 also came up during the trial of former Trump adviser Roger Stone.

According to evidence shown to the jury during the trial, Stone received an incoming call on August 1, 2016, from a phone number listed as “-1,” after speaking earlier that day with both Trump and Trump’s campaign advisor.

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However, it is uncertain if the calls came from one specific person and if the “-1” who spoke with Stone is the same “-1” who talked to Giuliani.