John Bolton is said to have been the most prolific note-taker at the top level of the White House.

Current and former senior administration officials told Axios that the former national security adviser probably has more details than any impeachment inquiry witness, so far, about President Trump's machinations on Ukraine.

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John Bolton is the impeachment inquiry's biggest wildcard. People around the president say they are worried about what notes Trump's former national security adviser has kept and when he might divulge them.

Why it matters: These sources, including both current and former senior administration officials, tell me that the former national security adviser was the most prolific note-taker at the top level of the White House and probably has more details than any impeachment inquiry witness, so far, about President Trump's machinations on Ukraine.

"Bolton was a voracious note-taker, in every meeting," said a source who attended numerous meetings with him.

While others sat and listened in meetings with Trump, Bolton distinguished himself by filling legal pads with contemporaneous notes on what was said in the room.

The intrigue: Bolton's lawyer, Chuck Cooper, caught the attention of impeachment investigators and administration officials on Friday with a provocative line he dangled in a letter to the House's general counsel: that Bolton "was personally involved" in "many relevant meetings and conversations that have not yet been discussed" in the impeachment testimonies.

Behind the scenes: This tease from Bolton's lawyer provoked this warning from a senior administration official:

"Typically, anything that could contain classified information is turned over to the White House for review when an employee departs. … One would hope Bolton has considered that before advertising that he has additional information."

"It could be that these are notes that the White House has already viewed," the official added. "But if not, it would mean Bolton deliberately concealed them during his offboarding, which could lead to legal repercussions depending on the contents."

To be clear, there is no evidence that Bolton has held onto classified information. But the unease inside the administration has been churning ever since staff learned that Bolton had signed a book deal to tell about his time working for Trump. Bolton's book deal is worth $2 million, per AP, which suggests he's willing to dish.

Between the lines: Unlike most of the impeachment witnesses so far, Bolton talked one-on-one with the president on numerous occasions. He has a degree of insight that the impeachment witnesses we've seen so far simply cannot offer.

Bolton and his former deputy Charlie Kupperman — both represented by Cooper — have asked a court to determine whether they should obey the White House's order not to testify or the House Democrats' request for their testimony.

The bottom line: All of this may come to nothing because House Democrats are rushing to vote on impeachment before the end of the year. Democrats have said they don't want to get sucked into a lengthy legal battle, and so they have not subpoenaed Bolton to testify. The courts, however, have shown they're willing to move faster than normal if Democrats were willing to delay their timeline and chase Bolton.

If House Democrats maintain their current stance — and there's every sign they will — it's a relief for administration officials who have been nervously contemplating what Bolton knows and what he's recorded in that mountain of legal pads.

Even if Bolton doesn't testify, he could still put out information before the election that's deeply damaging to Trump.

The big picture: Unlike current administration officials who may be motivated not to testify by their loyalty to President Trump, it seems more likely, based on Bolton's background, that he genuinely wanted the courts to resolve whether Congress or the White House has the power to compel or stop him from testifying.