President Donald Trump urged Sen. Lindsey Graham (R-SC) on Thursday to call former President Barack Obama to testify about his effort to investigate and unmask Trump’s former national security adviser, Gen. 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,” Trump wrote on Twitter. “He knew EVERYTHING.”

President Trump commented after Graham appeared on Fox & Friends vowing to get to the bottom of the Obama administration’s investigation of Gen. Michael Flynn:

The president expressed frustration at Graham for talking tough but failing to call members of the Obama administration to testify about their role in Flynn’s unmasking.

“Do it @Lindsey Graham, just do it. No more Mr. Nice Guy. No more talk!” he wrote.