It may be the most scrutinized day of the Nixon administration: June 30, 1971, the day that the Supreme Court decided in a 6–3 vote that the government had no legal right to prevent the New York Times from publishing the Pentagon Papers. The Papers’ revelations about Vietnam had infuriated Nixon and his inner circle, and the court decision led to the creation of the White House Special Investigations Unit —better known as “the Plumbers”—the team of dark-deed operatives who eventually conducted the Watergate break-in. Indeed, the first Plumber mission was to break into a Beverly Hills psychiatrist’s office that September, in hopes of finding damaging material about the Papers’ leaker Daniel Ellsberg. Nixon’s defense secretary, Melvin Laird, would later write, “If there was a single day that could be pinpointed when the Nixon presidency began to unravel, it was June 30.”

And yet for all the attention that June 30 has received, there is at least one detail that has not been discussed.

Recently, while researching the Nixon administration’s decision to go off the gold standard in August 1971, I came across some documents—portions of which were declassified only in 2013— that spotlight a different aspect of the White House attempt to smear Ellsberg. The documents are diary entries from White House Chief of Staff H. R. Haldeman. His June 30 entry recounts a debate within the administration about whether or not and how to release the Papers widely. Defense Secretary Laird argued that all the Papers should be released because friendly newspapers would, in Haldeman’s words, “carry the stories with the slant that we would want.” National Security Adviser Henry Kissinger argued for not releasing the Papers at all. In addition, Haldeman describes a “general agreement that there is very definitely a conspiracy here,” and some speculation about a possible chain of people who could have funneled the Papers to Ellsberg.

Then, Haldeman writes, President Nixon called him that evening at home, arguing that “we should not release the papers.” Haldeman reports that “the President wants to take a hard line and not cave in. He wants [Chief Domestic Advisor John] Ehrlichman to develop a plan on that basis. That we ought to start using Laird and his espionage unit on this, that we have to get all the information out.” Two days later, on July 2, Haldeman wrote in his diary that Nixon still wanted him to pursue the idea that Ellsberg was part of a broader conspiracy: “He had me call J. Edgar Hoover to push him on pursuing the conspiracy side, also wants me to talk with Laird about, about getting his spy unit on this.” (The phrases in italics were declassified on June 12, 2013. To my knowledge, no one has drawn attention to them until now.)

What do these phrases mean? Before the White House Plumbers convened, there was already a team inside the Pentagon assigned to examine leaks to the press. Given potential national security implications of any given leak, such investigations were reasonably common in defense and intelligence agencies—and they still are. Earlier that year, this team had tried to locate the source of secret Vietnam cables that columnist Jack Anderson—a constant administration nemesis—had obtained. (Among other embarrassments, Anderson had disclosed that the U.S. was spying on its own ally, South Vietnamese president Nguyen Van Thieu.) The Anderson investigation was exhaustive; nearly 200 people were interviewed, though the culprit was never found. In his book Poisoning the Press: Richard Nixon, Jack Anderson, and the Rise of Washington’s Scandal Culture, Mark Feldstein quotes the former FBI investigator who conducted the Anderson investigation as saying that the administration made it a high priority: “All doors were opened for us. If we needed planes, ships, or donkeys, we had them.”

Anderson later mocked the inquiry as the work of the “Pentagon Plumbers.” But a key difference between the White House and the Pentagon teams is that Defense’s investigations, while grueling, were presumably by the book, while the White House Plumbers specialized in illegal break-ins and dirty tricks.

What the long-redacted Haldeman diary phrases show, then, is that when the Supreme Court Pentagon Papers decision forced a response, Nixon’s first instinct was to go to an already-existing Plumber-style unit in the Pentagon.

This little-discussed connection between the Pentagon’s 1971 investigation into journalistic leaks and the founding of the White House Plumbers raises several intriguing historical questions. First, why did government archivists initially delete these phrases from Haldeman’s diary entry? Is it because the breadth of illegal shenanigans in that administration was even greater than we have previously realized?

Another question is, do we have a thorough, truthful account of the defense department’s investigation into the origins of the Pentagon Papers leak and who Ellsberg’s associates were? One possibility is that Nixon and his aides wanted to cross investigative lines that Laird—who at this stage was something of an administration outsider, and happily so—refused to do.

Nixon during this period became obsessed with breaking into the office of the Brookings Institution, believing that there were incriminating documents there, in a safe. Laird’s authorized biography, published in 2008, discusses the June 30 meeting and the attempts to dig up dirt on Ellsberg. Laird says that he began to fear that Nixon’s aides “were getting a little wild,” a vague but suggestive phrase. But how credible is Laird? He insists he was not in the room on June 30 at the moment when breaking into Brookings was discussed, even though the White House record does not show him leaving.

But perhaps most strikingly, these discoveries serve as a reminder that the often achingly slow release of sensitive government material can deny us a complete historical understanding of events for decades, perhaps forever. The book publication of Haldeman’s diary in 1996 was widely and correctly heralded as a breakthrough in the historical record. Nonetheless, in that published version, the June 30, 1971, entry omits entirely any reference to Nixon’s phone call.

In a 1996 article titled “Researchers’ Nightmare: Studying the Nixon Presidency,” historian Joan Hoff wrote of Haldeman’s diaries: “It is still quite possible that when certain documents removed by the representatives of Nixon or his closest aides…are completely opened, that the circle of those with knowledge of various aspects of the break-ins and cover-up will ultimately widen to include the men closest to Nixon.” We might be a little closer to that point now, but then again, we may never truly get there.