It's the memo that everyone is talking about, but no-one can (really) talk about.
Depending on whom you ask, it will either explode the entire basis for the Trump-Russia investigation or is an inflammatory collection of rumours and conjecture.
Donald Trump Jr has tweeted about it. More than 200 members of Congress have seen it. A social media hashtag - #releasethememo - has been pushed by interested parties including, it appears, a sizable number of pro-Russian Twitter bots.
So what, exactly, is this memo?
The document was written by staffers working for Devin Nunes, the chair of the House Intelligence Committee, which is holding hearings on Russian meddling in the 2016 presidential election and, seemingly of equal interest to the congressman, investigating allegations of anti-Trump bias in the US intelligence community.
Mr Nunes made headlines last March when he visited the White House and then publicly announced that some members of Donald Trump's post-election team had been the subject of "incidental surveillance" by US intelligence agencies. He later had to step down from overseeing the Russian hearings because he was under investigation by the House Ethics Committee for possibly sharing classified intelligence with the White House. He has since been cleared in the matter by the ethics panel.
The four-page memo is said to draw on highly classified information about FBI surveillance practices that, according to many of the Republicans who have read it, were abused by the intelligence community in order to undermine Mr Trump's campaign and, subsequently, his presidency.
The memo reportedly asserts that the decision to begin surveillance of Trump campaign foreign policy adviser Carter Page was based on the so-called Steele dossier - the collection of largely unsubstantiated raw intelligence gathered by former British intelligence agent Christopher Steele. That set the ball rolling for broader investigation into possible ties between the Trump campaign and the Russian government, an inquiry that continues, under the direction of special counsel Robert Mueller, to this day.
Members of the intelligence community dispute this claim and insist that Page was on the counter-intelligence radar long before the dossier research, which was funded by anti-Trump conservatives and later Democrats, was ever circulated.
What else might be in it?
The memo may also reportedly contain - or reference - evidence of anti-Trump sentiment from high-level FBI officials, possibly including Deputy Director Andrew McCabe and Peter Strzok, the top FBI Russia expert involved in the bureau's 2016 election investigations and who later moved over to Robert Mueller's special counsel investigation.
Strzok has been in the headlines for text messages he exchanged in 2016 with an FBI colleague with whom he was having an extramarital affair. The messages, many of which have already been publicly released, show the two criticising candidate Trump, as well as other politicians and Justice Department officials.
Conservative commentators and some politicians have expressed concern that other messages, dating from late 2016 through the early days of the Trump presidency, were not preserved by the FBI due to misconfigured mobile phones.
Then there are rumours that the memo might also cast Rod Rosenstein, who as deputy attorney general is overseeing the Justice Department Russia investigation, in a less-than-favourable light. If all this sounds rather vague, it's because the memo has yet to be made public. Lots of people are talking about what it means, but few are giving any specific details about what it contains.
What are Republicans saying?
As the "secret" memo circulates through the halls of Congress, many conservatives are saying it contains explosive revelations.
Iowa Congressman Steve King has compared the allegations in the memo to the scandal that brought down President Richard Nixon, saying they're "worse than Watergate".
"I've read the memo," says Florida Republican Congressman Matt Gaetz, who has been one of the sharpest critics of the Mueller investigation. "It has surprised and horrified many of us."
He said that the memo will not just lead to firings within the FBI, but also criminal prosecution.
Other Republicans have downplayed the implications of the memo.
"I think there's a lot of paranoia around here when it comes to that topic," John Cornyn, a top Senate Republican, told Politico.
While Donald Trump himself has not commented on the memo - although Gaetz reportedly discussed it with the president during a meeting - one of his sons has taken up the cause.
"It sounds pretty troubling to me," Donald Trump Jr told Fox News. "The interesting thing about it is the only collusion, the only shadiness, the only thing that's been discovered is what the prior administration, the [Democratic National Committee] and all of them have done."
Do congressional Democrats see it this way?
Hardly. Adam Schiff, the ranking Democrat on the House Intelligence Committee, says the memo is based on cherry-picked information presented in a "profoundly misleading" way.
"Rife with factual inaccuracies and referencing highly classified materials that most of Republican Intelligence Committee members were forced to acknowledge they had never read, this is meant only to give Republican House members a distorted view of the FBI," Schiff said in a press statement.
Jarod Nadler of New York, the ranking member on the House Judiciary Committee, complained in a letter to committee chair Bob Goodlatte that the FBI and the Justice Department has not been able to see the memo in order to respond to the allegations.
Mr Schiff and Dianne Feinstein, the ranking Democrat on the Senate Intelligence Committee, have also noted that the hashtag conservatives are using to push for public dissemination of the document - #releasethememo - is being promoted by social media accounts linked to Russian influence operations. In a letter to Facebook and Twitter executives, they note data from a social media watchdog group that showed that particular hashtag was by far the most tweeted by Russian bots.
"If these reports are accurate, we are witnessing an ongoing attack by the Russian government through Kremlin-linked social media actors directly acting to intervene and influence our democratic process," they write. "This should be disconcerting to all Americans, but especially your companies, as once again, it appears the vast majority of their efforts are concentrated on your platforms."
Will the memo be released?
Probably. As president, Mr Trump has the power to declassify any document he wishes. The House Intelligence Committee has voted to release the memo, which Mr Trump must accept or reject. If he signs off, the full House of Representatives could meet in closed session to vote on the matter. Or the president could take the committee approval as sufficient to justify ordering the release immediately.
Sixty-five members of the House of Representatives have signed a letter urging the memo's publication.
A member of Congress with a particularly good memory could even recite details of the memo into the congressional record, free from legal jeopardy.
Then there's the possibility that the memo - which is being kept in a secure location - could end up anonymously leaked to the press. Wikileaks - which the US intelligence agencies concluded collaborated with Russian interests during the 2016 election - has offered $1m for anyone who provides it with a copy of the document.
Democrats in Congress - and some Republicans - have resisted efforts to make the memo public, citing the highly classified intelligence on which it is based.
"The documents that supposedly inform these talking points are highly classified," the members of the House Intelligence Committee said in a joint statement. "And they will not be made public, making it impossible for the few members who have seen the documents to explain the flaws and misstatements contained within the talking points without disclosing our most closely held intelligence sources and methods."
The FBI and Mr Trump's own Justice Department have objected to the memo's release. An FBI spokesperson released a statement saying "we have grave concerns about material omissions of fact that fundamentally impact the memo's accuracy".
For now, the memo remains a shadow. At some point it may see the light of day, but it seems unlikely that the dispute over its meaning and political potency will be put to rest even then.
The investigation into the Trump campaign and Russian election meddling that has resulted in several high-profile indictments and draws closer to the president himself is becoming increasingly mired in a partisan melee. Given the stakes, the heat will only grow more intense in the days ahead.