A new book by Andrew Weissmann, who served on Robert Mueller’s Special Counsel team investigating Russian election meddling in 2016, says the group pulled punches during its investigation in the face of repeated threats from the president to shut down the probe. The failure to use the full extent of the prosecutors’ authority to get to the bottom of Trump’s Russia links ultimately let Trump off the hook, kept the American people in the dark, and left Weissmann to lament: “We could have done more.” In Weissmann’s insider account of the two-year Russia investigation, Where the Law Ends: Inside the Mueller Investigation, the former prosecutor goes about cataloging the internal thinking, strategy, and shortcomings of Mueller’s tightlipped prosecutorial team. After years of presidential grandstanding and right-wing misinformation, Weissmann’s telling of how the case against the president was made creates, for the first time, a historical record of what went on behind the scenes of the monumental investigation.

While Weissmann outlines some of the strategic failures, the real takeaway is that the Trump threats to shut down the special counsel were effective: They deeply affected how the investigation was run, Weissmann writes, so much so that the Mueller team went to great lengths to avoid angering the president and his backers in the right-wing media. The Mueller team, for instance, chose not to subpoena the president to compel him to sit for an interview; the special counsel also declined to subpoena Trump’s financial records and corral key documents from the Trump Organization. In each case, Weissmann writes, the decisions were made out of fear that Trump would pull the plug in response. So the investigators declined to compel Donald Trump Jr.’s testimony and did not interview Ivanka Trump, despite both Trump children interacting with Russian-linked figures while playing key roles on the campaign trail.

While Weissmann acknowledges the responsibility for the investigation ultimately falls on Mueller’s shoulders, he places much of the blame on Mueller deputy Aaron Zebley for bending over backward to fly under the radar of the White House. “Aaron had a way of gaslighting you, of making you question your own reality, that you were being too aggressive in your drive to pursue leads and push harder—like you were not enough of an adult,” Weissmann writes. In the end, the strategy of gathering as much evidence and data as possible while not arousing too much concern from the White House proved ineffective, as did Mueller’s approach of not stating plainly what the president was guilty of, instead leaving that to Congress and (potentially) future investigations.

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Now allowed to editorialize about the probe and the process, the New York Times notes, Weissmann “elevates particular details—for example, emphasizing that the same business account that sent hush payments to an adult film star who alleged an extramarital affair with Mr. Trump had also received ‘payments linked to a Russian oligarch.’ ” From the Times:

The book builds toward investigators’ discovery that Mr. Manafort had shared internal campaign polling data with Mr. Kilimnik, who flew to the United States to meet with Mr. Manafort during the campaign, asking whether Mr. Trump would permit a peace plan for Russia to essentially take over all of eastern Ukraine.

But while admitting this much, Mr. Manafort—seeing the dangle of a potential pardon from Mr. Trump—refused to cooperate further. Investigators did not obtain any final puzzle pieces and lacked the evidence to charge anyone in the campaign with a criminal conspiracy involving Russia’s covert electoral assistance.

“It would seem to require significant audacity—or else, leverage—for another nation to even put such a request before a presidential candidate,” Mr. Weissmann wrote of Mr. Kilimnik’s request. “This made what we didn’t know, and still don’t know to this day, monumentally disconcerting: Namely, why would Trump ever agree to this? Why would Trump ever agree to this Russian proposal if the candidate were not getting something from Russia in return?”

Without financial records to seal the deal or compelled testimony to out wrongdoing or, minimally, flip witnesses that were eyeing presidential pardons, the investigation failed to deliver on its promise to plumb the darkest depths of Trump’s Russia links. “We still do not know if there are other financial ties between the president and either the Russian government or Russian oligarchs,” Weissmann writes. “We do not know whether he paid bribes to foreign officials to secure favorable treatment for his business interests, a potential violation of the Foreign Corrupt Practices Act that would provide leverage against the president. We do not know if he had other Russian business deals in the works at the time he was running for president, how they might have aided or constrained his campaign, or even if they are continuing to influence his presidency.”

The Mueller investigation refused to go all the way, to use its authority to leave no stone unturned in the pursuit of truth; it described what was plain in abstraction, and in doing so did left the American public in perhaps a worse off position than they started—nowhere.