Benjamin Wittes speaks at an event in D.C. (Screenshot via C-SPAN)

A "good friend" of former FBI Director James Comey indicated there is cause for concern in Deputy Attorney General Rod Rosenstein's resignation letter.

In announcing his resignation on Monday, Rosenstein thanked President Trump for "the courtesy and humor you often display in our personal conversations.”

While Comey has remained silent in the 24 hours since the letter went public, Benjamin Wittes, editor-in-chief of Lawfare, reacted to that passage with uneasiness. "The deputy attorney general shouldn't be having 'personal conversations' with the president," he said.

The deputy attorney general shouldn't be having "personal conversations" with the president. https://t.co/3OezMaxfhu — Benjamin Wittes (@benjaminwittes) April 29, 2019

Rosenstein appointed Robert Mueller to be special counsel in charge of the Russia investigation just eight days after Trump fired Comey in May 2017 and oversaw its operations. He was also a witness in Mueller's investigation.

Rosenstein's long-expected resignation, effective May 11, comes days after the publication of a bombshell Washington Post report about how Rosenstein fought to not get fired by tweet last fall after it came to light that he told Justice Department officials about wearing a “wire” to record conversations with Trump and that he had discussed invoking the 25th Amendment against the president to remove him from office. “I give the [Mueller] investigation credibility,” Rosenstein said in a call with the president. “I can land the plane.”

House Intelligence Committee Chairman Adam Schiff, D-Calif., says he would like Rosenstein to come in for a hearing. "Rod Rosenstein was a witness. Rod Rosenstein was a participant in what led to the firing of James Comey, and he should not have had any role in whether the obstruction case rose to the level of a prosecutable crime," Schiff said at a Washington Post Live event on Tuesday.