(CNN) As President Donald Trump's reaction to special counsel Robert Mueller grows more irate by the day, attorneys on both sides sat down last week in a rare face-to-face discussion about the topics investigators could inquire of the President. It was the first in-person meeting after several weeks of informal discussions between the two sides, according to two sources familiar with the talks.

Mueller's team added granularity to the topics it originally discussed with the defense team months ago, like the firing of FBI Director James Comey, according to one of the sources. This time around, for instance, the prosecutors said they would ask about Attorney General Jeff Sessions' involvement in the Comey dismissal and what Trump knew about national security adviser Michael Flynn's phone calls with then-Russian Ambassador Sergey Kislyak in late December 2016.

The meeting makes clear that Mueller's investigation into contact between Russians and the Trump campaign and other criminal matters isn't likely to end anytime soon and still may focus on Trump and what he knew. The meeting and its revelations also have unleashed a new level of Trump's public hostility toward Mueller, even while some of the President's advisers show a willingness to negotiate Trump's testimony.

The President's attorneys sent the special counsel a summary of evidence they had turned over to prosecutors already, a practice they've followed multiple times throughout the investigation. Mueller himself didn't attend the meeting. But prosecutors including former Watergate prosecutor James Quarles III gave Trump's lawyers enough detail that the President's team wrote a memo with possible questions they expect to be asked of him.

One source familiar with the matter said the President has vacillated on agreeing to an interview in recent months. Trump was interested in speaking with Mueller's team if the probe would end soon, as his lawyers had long promised, another source said. But once the President realized Mueller's work isn't nearing a conclusion, he's become more agitated and has lashed out on Twitter.