WASHINGTON — How did WikiLeaks, the anti-secrecy group used as a conduit by Russian intelligence to distribute hacked emails, come to release a trove of documents just when Donald Trump’s candidacy seemed to need it most? Why did Trump’s campaign chief share internal polling data with a man tied to Russian intelligence? Did the president encourage his lawyer to lie to Congress?

After nearly two years of work, investigators working for special counsel Robert Mueller said they still don't know.

Mueller's office probed deeply into Trump's campaign and the frantic first months of his presidency, and produced a final report brimming with inside details. But its work left some of the inquiry’s most perplexing questions frustratingly unanswered.

Mueller's report revealed in staggering detail efforts by the Russian government to help Trump win the presidency, his campaign’s interest in benefiting from that effort, and a series of steps by Trump to thwart the investigations that followed. Prosecutors ultimately did not conclude that Trump had committed a crime.

The investigation gathered information from hundreds of witnesses. Prosecutors and FBI agents served more than 2,800 grand jury subpoenas and 500 search warrants, yielding documents, electronic communications and troves of other records. But even that massive collection of evidence left investigators with some dead-ends.

Prosecutors wrote that they were sometimes hamstrung by witnesses who lied to them, or provided “incomplete” information. Some evidence they wanted was protected by attorney-client privilege, and some was in the hands of people who live outside the United States and were largely beyond their reach. And some Trump aides deleted their electronic messages before prosecutors could review them.

So Mueller issued his conclusions with a caveat: “Given the identified gaps, the Office cannot rule out the possibility that unavailable information would shed additional light on (or cast in a new light) the events described in this report.”

Here is what they couldn’t find:

A well-timed WikiLeak

Mueller’s investigation concluded that in 2016, Russian intelligence operatives hacked into Democratic political organizations and stole huge troves of emails, many of them embarrassing to Trump’s political opponents. The intelligence service passed them to WikiLeaks to make them public, a move “designed and timed to interfere with the 2016 U.S. presidential election and undermine the Clinton Campaign.”

Investigators said Trump’s campaign “showed interest” in the documents being distributed by WikiLeaks, and worked to learn what other information the group might possess.

One release of records came at an especially perilous moment for Trump’s campaign. On Oct. 7, 2016, The Washington Post published a leaked Access Hollywood outtake in which Trump boasted in graphic language about sexually assaulting women, prompting an instant backlash. Less than an hour later, WikiLeaks published a batch of emails the Russians had stolen from Clinton’s campaign chairman, John Podesta.

Prosecutors focused on the timing of that and other releases, but said they were “unable to resolve” a question about the Oct. 7 release.

Michael Cohen’s lies to Congress

Trump’s former personal lawyer, Michael Cohen, has pleaded guilty to lying to Congress about efforts to erect a Trump Tower in Moscow. Cohen told lawmakers in a written statement that the project ended in early 2016, but it actually continued for several more months, until the point when Trump had virtually secured the Republican nomination. Cohen pleaded guilty last year to lying to Congress about those plans.

Mueller’s report said Cohen had circulated that written statement to lawyers participating in a joint defense agreement with Trump, including Trump’s new personal counsel before he submitted it to Congress. Telephone records showed that Cohen and the president’s personal lawyer — whom the report did not name, but Cohen said was Jay Sekulow — spoke “almost daily” in the period between when Cohen emailed a draft of his testimony and when he finally sent it to Congress.

Cohen testified to a House committee in February that Trump had indirectly encouraged him to lie to lawmakers. "Mr. Trump did not directly tell me to lie to Congress," Cohen said. "That's not how he operates. He would look me in the eye and tell me there's no business in Russia and then go out and lie to the American people by saying the same thing. In his way he was telling me to lie about it."

Prosecutors wrote in Mueller’s report that they could not corroborate that claim, in part because the president’s lawyers declined to speak with them about privileged conversations. “The absence of evidence about the President and his counsel’s conversations about the drafting of Cohen’s statement precludes us from assessing what, if any, role the President played,” the wrote.

It is not unusual for privilege questions to frustrate investigations, said Randall Eliason, a George Washington University law professor and former federal prosecutor. "I think it’s information that would be good for the public to know, but we operate in the confines of the legal system that recognizes attorney-client privilege," he said.

Internal polling and the GRU

Mueller’s investigation concluded that Trump’s former campaign chairman, Paul Manafort, repeatedly shared the campaign’s internal polling data with a Russian associate “assessed by the FBI to have ties to Russian intelligence.” Manafort and his deputy, Rick Gates, both provided “internal polling data and other updates” to the man, Konstantin Kilimnik, while they were serving on Trump’s campaign.

Both Manafort and Gates have been convicted of crimes related to their illicit work for a pro-Russian political faction in Ukraine. Manafort was sentenced to a total of 7.5 years in prison.

Prosecutors wrote that Manafort thought sharing the information would be “good for business,” and perhaps help him settle a debt he was owed by a Russian oligarch. But prosecutors, who had accused Manafort of lying to them repeatedly, mistrusted his explanation. They said they had “limited ability to gather evidence on what happened to the polling data” after Manafort sent it. They said they “did not identify a connection between Manafort’s sharing polling data and Russian interference in the election.”

Ultimately, prosecutors said they “could not reliably determine Manafort’s purpose in sharing internal polling data.”

Carter Page’s visit to Moscow

Federal agents secretly told a surveillance court in late 2016 that one of Trump’s foreign policy advisers, Carter Page, “has been collaborating and conspiring with the Russian government” in its efforts to intervene in the 2016 election. The statement was part of a request to eavesdrop on Page, who had recently left Trump’s campaign.

Page came under scrutiny in part because of an infamous “dossier” compiled by former British spy Christopher Steele, who was paid indirectly by Clinton’s campaign and the Democratic National Committee to dig up information about Trump’s ties to Russia. Steele provided information to the FBI about a trip Page took to Moscow in July 2016. During that trip, he told a campaign official in an email that Russia’s deputy prime minister had “expressed strong support for Mr. Trump.”

In a passage of Mueller’s report that is largely blacked-out, prosecutors wrote that they were “unable to obtain additional evidence or testimony about who Page may have met or communicated with in Moscow; thus, Page’s activities in Russia – as described in his emails with the campaign – were not fully explained.”

What did the president know?

Mueller’s office spent more than a year seeking to interview Trump about Russian election interference and his attempts to thwart their investigation, at one point telling him “it is in the interest of the Presidency and the public for an interview to take place.” Ultimately, Trump agreed to answer only written questions, and only about “certain Russia-related topics.”

Prosecutors asked questions about subjects ranging from efforts to build a Trump Tower in Moscow to a meeting his aides attended at Trump Tower in New York with a Russian lawyer offering damaging information on Clinton. Trump’s lawyers turned in his answers in November.

Prosecutors found his answers unsatisfying. More than 30 of them included statements that Trump did not remember the answer to the question. Others were “incomplete or imprecise.” They replied by again seeking to interview Trump, but his lawyers declined.

Mueller’s office considered issuing a grand jury subpoena to force the president to testify but decided not to. Instead, they said, they “determined that the substantial quantity of information we had obtained from other sources allowed us to draw relevant factual conclusions on intent and credibility.”

Contributing: Kristine Phillips