“Having to read a footnote,” the playwright Noël Coward once remarked, “resembles having to go downstairs to answer the door while in the midst of making love.” So why then, in his otherwise bare-bones letter to Congress summarizing the Mueller report, did Attorney General William Barr include a technical, 44-word footnote carefully defining the term coordination—the only term defined in the letter?

Was the footnote just an afterthought, as some have treated it? Or does it provide a clue as to why Mueller ultimately concluded he could not establish a Trump-Russia conspiracy?

The footnote appears in the section of Barr’s letter on Russian election interference, which quotes from Mueller’s report only twice. The first quote summarizes Mueller’s conclusion, stating, “[T]he investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities.” Barr then clarifies in a footnote how Mueller defined coordination:

In assessing potential conspiracy charges, the Special Counsel also considered whether members of the Trump campaign “coordinated” with Russian election interference activities. The Special Counsel defined “coordination” as an “agreement—tacit or express—between the Trump Campaign and the Russian government on election interference.”

On first blush, the footnote’s purpose seems straightforward: distinguishing conspiracy from coordination. But that’s not the case. Because a conspiracy also requires a tacit or express “agreement,” the footnote indicates that Mueller defined coordination consistently with conspiracy, even borrowing its definition from conspiracy law. Barr therefore did not add the footnote to distinguish the two terms.

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Another potential explanation is that Barr included the footnote to be thorough. Lawyers have a habit of defining terms and citing extraneous details, so Barr might have added the footnote out of lawyerly instinct. But that contrasts with the rest of the letter, which, at about four pages, is anything but exhaustive. On every other topic, Barr provides only a vague summary of Mueller’s conclusions without detailing the inquiries and legal definitions underlying them, or any of Mueller’s descriptions of the facts he discovered. Barr defines no other terms in the letter, includes no other substantive footnotes, and quotes from the report only sparingly.

Was the footnote just an afterthought, as some have treated it?

So why then, in a letter devoid of details that he knew would be parsed for meaning, did Barr feel the need to include a lengthy footnote that expressly defined coordination as a tacit or express “agreement”?

The most likely explanation is that this definition was central to Mueller’s conclusion that he could not establish “coordination,” and thus it was necessary to put that conclusion in context. Had Barr not clarified that “coordination” meant “agreement,” the public might assume that coordination meant something less formal, like cooperation or interaction, and infer that Mueller found no new evidence of such conduct. Which means if Mueller did find such evidence and it became public, Barr’s letter could be deemed misleading. By including the footnote, Barr gave himself cover by clarifying that Mueller did not clear Trump of coordination in the everyday sense of the word ­but in a more legalistic sense that requires some form of “agreement.”

This suggests that there may have been enough evidence implicating Trump to make this distinction material—material enough for Barr to include it in his otherwise perfunctory letter. And this in turn suggests that Mueller’s findings regarding collusion might not be the “total exoneration” for Trump that he has claimed. Mueller’s inability to establish Russian coordination could have equally stemmed from his legal concerns about proving an “agreement” than from a genuine lack of evidence of collusive behavior on the part of the Trump campaign. That would certainly explain why Barr felt inclined to preview the issue in the letter’s sole substantive footnote.

This is consistent with what I wrote in Slate in February about the difficulties Mueller would face in proving a Trump-Russia “agreement,” even if he had circumstantial evidence of collusion:

Prosecutors [will] need to show that Trump and Russia reached some semblance of an actual agreement, which the law defines flexibly as “a unity of purpose or a common design and understanding, or a meeting of minds.” […] If after all the evidence is presented, Trump can offer a plausible explanation for his conduct that does not involve an agreement with Russia, prosecutors will have failed to “exclude the possibility” that Trump and Russia acted independently, making a conviction unlikely. To avoid this, Mueller will need enough circumstantial evidence to eliminate any non-collusive explanations for Trump’s conduct, including the possibility that Trump and Russia acted “consciously parallel.” Doing so will require more evidence that what is publicly known today.

Until Barr releases more details from the report, it will be impossible to know just how close Mueller came to establishing an agreement under this test. Mueller might have uncovered little new evidence of coordination and never seriously considered bringing charges. But he also could have uncovered significantly more evidence and still determined it was insufficient to establish an agreement. Given Thursday’s reporting from the New York Times that Mueller’s report exceeds 300 pages, it seems at least plausible that Mueller discovered important evidence the public is yet to learn.

These questions hold potentially significant political implications. If Mueller concluded that despite additional evidence he was unable to establish coordination because of the difficulties of proving an “agreement,” that would place Trump on a far weaker political footing than if Mueller cleared Trump because he found no new evidence of wrongdoing. Barr’s decision to include the footnote suggests that the “agreement” requirement did factor in Mueller’s decision-making and that additional evidence exists, but we will not know for sure until the full report is released.