It’s the nature of a criminal investigation that people aren’t going to own up to everything they did. That’s why there’s an investigation: to figure out what happened despite people not wanting to admit the truth.

One of the themes in the partially redacted report from special counsel Robert S. Mueller III's team is that, in a number of key areas, they were left unable to determine the facts surrounding a particular issue. In both of the report's volumes — the first addressing possible conspiracy between Russia and President Trump's 2016 campaign and the second addressing possible obstruction of justice — there are examples of how and where Mueller's investigators hit brick walls.

The report itself provides an overview of the problems Mueller's team faced. (The report refers to Mueller's team as “the Office.")

"Some individuals invoked their Fifth Amendment right against compelled self-incrimination and were not, in the Office's judgment, appropriate candidates for grants of immunity,” it states. Internal Justice Department guidelines blocked outreach to some witnesses. Other material was potentially subject to attorney-client privilege and screened before being given to Mueller's team.

“Even when individuals testified or agreed to be interviewed,” the report continues, “they sometimes provided information that was false or incomplete. . . . And the Office faced practical limits on its ability to access relevant evidence as well — numerous witnesses and subjects lived abroad, and documents were held outside the United States.”

The most obvious example comes in Mueller’s efforts to question Trump himself. After months of negotiation, Mueller’s investigators agreed to accept written responses to questions in lieu of a sit-down interview with the president. What they got was, in the report’s phrasing, “inadequate”: An extensive list of “I don’t recall”s that left the vast majority of the team’s questions essentially unanswered.

They considered subpoenaing Trump, which would have kicked off a huge, historic legal battle. They decided against that step, because by then their investigation had “made significant progress and had produced substantial evidence for our report."

"[T]he substantial quantity of information we had obtained from other sources allowed us to draw relevant factual conclusions on intent and credibility,” the report states, “which are often inferred from circumstantial evidence and assessed without direct testimony from the subject of the investigation."

With that decision, what they got from Trump was all they got from Trump.

Earlier this year, we learned of an interaction between a senior Trump campaign official and a Russian linked to that country’s intelligence service that a Mueller attorney said went “very much to the heart of what the special counsel’s office is investigating.”

Over the course of 2016, beginning in the spring or early summer and extended past August, then-campaign chairman Paul Manafort shared poll data with his colleague Konstantin Kilimnik, someone that even Manafort’s business partner Rick Gates thought was a “spy,” according to the report. When this was first reported, it wasn’t clear why Manafort was sharing the data: Was it part of his unending financial hustle? Or was it part of an effort to give Russia a heads up on where the campaign was headed?

We still don't know.

"Because of questions about Manafort's credibility and our limited ability to gather evidence on what happened to the polling data after it was sent ta Kilimnik,” the report states, “the Office could not assess what Kilimnik (or others he may have given it to) did with it.” Mueller's team didn't find evidence that the data was connected to Russia's interference effort and they didn't determine that Manafort had coordinated with Russia's efforts in other ways.

Kilimnik was apparently not interviewed by Mueller’s team (though he was later indicted on charges of obstruction). That he wasn’t interviewed isn’t entirely surprising, particularly considering the report excerpt above. As a resident of Ukraine, he’s a good example of someone outside of the Mueller team’s reach.

Less obvious is why Mueller didn’t interview or subpoena key Americans. Donald Trump Jr., for example, never spoke with Mueller’s team, despite being at the center of a number of critical issues, including the proposed Trump development project in Moscow, interactions with WikiLeaks and, of course, the June 2016 meeting at Trump Tower. Mueller’s team did subpoena others, including Andrew Miller, a former associate of Trump’s longtime adviser Roger Stone who was held in contempt last year for not responding to the subpoena.

An additional part of the problem in questions about Kilimnik was that Gates and Manafort worked in real-time to cover their tracks. The report indicates that Gates, following Manafort’s instructions, “deleted the communications [with Kilimnik] on a daily basis.” The pair communicated over WhatsApp, a message service that’s encrypted from end-to-end, making it harder for investigators to obtain records.

Gates and Manafort weren’t the only ones who seemed to be deleting data. Mueller’s investigators focused on a January 2017 meeting in the Seychelles between Blackwater founder Erik Prince and a Russian official named Kirill Dmitriev. Prince was friends with then-Trump adviser Stephen K. Bannon (in addition to being Education Secretary Betsy DeVos’s brother) and, according to Mueller’s team, discussed the meeting with Bannon.

"Prince's phone contained no text messages prior to March 2017, though provider records indicate that he and Bannon exchanged dozens of messages,” the report states. “Prince denied deleting any messages but claimed he did not know why there were no messages on his device before March 2017. Bannon's devices similarly contained no messages in the relevant time period, and Bannon also stated he did not know why messages did not appear on his devices."

No messages on the phones means no documentation for investigators to peruse. At other points, it's not clear why Mueller's team couldn't get answers to questions.

In December 2016, then-national-security-adviser-designate Michael Flynn spoke with then-Russian Ambassador Sergey Kislyak to recommend that Russia not respond in kind to sanctions imposed by the administration of President Barack Obama. Russian President Vladimir Putin decided against such a response, and Kislyak later credited Flynn for that action.

When that conversation became public in early 2017, Trump asked Flynn’s deputy K.T. McFarland to release a statement stating that he hadn’t directed Flynn to make that request to Kislyak. She declined, because “she did not know whether that was true,” the report states.

Mueller's team couldn't determine that, either.

"Although transition officials at Mar-a-Lago had some concern about possible Russian reactions to the sanctions,” the report states, referring to Trump staffers who were at the then-president-elect's Florida resort, “the investigation did not identify evidence that the President-Elect asked Flynn to make any request to Kislyak."

That’s odd, because Mueller’s team asked Trump that question specifically in their written questions. But Trump appears to have simply not responded.

Here's the section in which the question is asked.

Here’s Trump’s response, which addresses only the first part of the question.

Mueller's team appears not to have followed up.

As noted above, though, Mueller’s team appeared to have believed they had enough information from Trump to at least assess the question of obstruction of justice.

“[I]f we had confidence after a thorough investigation of the facts that the President clearly did not commit obstruction of justice, we would so state,” the report concludes. “Based on the facts and the applicable legal standards, we are unable to reach that judgment.”

“Accordingly,” it continues, “while this report does not conclude that the president committed a crime, it also does not exonerate him.”

In at least that sense, Mueller got all the answers he believed he needed.