President Donald Trump on Thursday, May 14, amplified the pressure on Sen. Lindsey Graham (R-S.C.) to call former President Barack Obama to testify amid new developments in the case of then-incoming National Security Adviser Michael Flynn.

“If I were a Senator or Congressman, the first person I would call to testify about the biggest political crime and scandal in the history of the USA, by FAR, is former President Obama. He knew EVERYTHING,” President Trump tweeted. “Do it @LindseyGrahamSC, just do it. No more Mr. Nice Guy. No more talk!”

The president’s demand came as it was revealed that top-ranking figures in Obama administration, including then-Vice President Joe Biden, then-FBI Director James Comey, then-CIA Director John Brennan, then-Director of National Intelligence James Clapper, and Obama’s then-chief of staff Denis McDonough, were involved in Flynn’s unmasking during the presidential transition period.

Graham criticized the Obama administration over the unmasking revelations, calling it “deeply troubling and smell of politics, not national security.” But he expressed reluctance to go so far as calling the former president to testify, saying in a statement that he’s “greatly concerned about the precedent that would be set by calling a former president for oversight.”

Nevertheless, Graham promised to “perform oversight” of the these unmasking requests, noting that he wants to know “how many unmasking requests were made, if any, beyond General Flynn regarding members of the Trump campaign team, family, or associates.”

Graham announced Thursday that the Senate Judiciary Committee will begin holding “multiple, in-depth congressional hearings” in early June to review the FBI’s Crossfire Hurricane investigation and issues related to Flynn.

“Our first phase will deal with the government’s decision to dismiss the Flynn case as well as an in-depth analysis of the unmasking requests made by Obama administration officials against General Flynn,” Graham said. “We must determine if these requests were legitimate.”

Graham said that later in the summer, the committee will dig further into the Justice Department inspector general’s (IG) report on the FBI’s abuse of the Foreign Intelligence Surveillance Act (FISA) against Carter Page, the former Trump campaign aide. 

The IG report indicated that the FBI committed “17 significant errors and omissions” in its applications against Page. Footnotes in the IG report, which were released in recent weeks showed that the FBI had received evidence of Russian disinformation in the Steel dossier that they used to obtain surveillance applications. 

“Finally, we will look at whether Robert Mueller should have ever been appointed as special counsel.  Was there legitimate reason to conclude the Trump campaign had colluded with the Russians?” he added.