WASHINGTON -- Judging by the numbers, special counsel Robert S. Mueller III left no stone unturned in the Russia investigation. His office issued more than 2,800 subpoenas, carried out almost 500 search warrants and spoke with roughly 500 witnesses.
But as his 448-page redacted report makes clear, some answers -- and some witnesses -- remained outside his grasp. "The investigation did not always yield admissible information or testimony, or a complete picture of the activities undertaken by the subjects of the investigation," Mueller wrote.
A mix of factors -- including uncooperative witnesses and deleted text messages -- left blind spots. Here are some of the gaps in the investigation:
-- THE PROFESSOR'S TIP
George Papadopoulos was a young foreign policy adviser for Trump's campaign when a Maltese professor with Russian connections told him in April 2016 that Moscow had "dirt" on Hillary Clinton in the form of thousands of emails. The tip came before WikiLeaks had released any emails hacked by Russian operatives.
Papadopoulos shared the information with an Australian diplomat, who told the FBI, and the Greek foreign minister at the time. Mueller's team wanted to know if Papadopoulos also informed the Trump campaign.
The report said Papadopoulos, who ultimately pleaded guilty to lying to investigators, sometimes "wavered" in his recollections. "Campaign officials ... (have) stated, with varying degrees of uncertainty, that he did not tell them," it added. "No documentary evidence, and nothing in the email accounts or other communications facilities reviewed by the Office, shows that Papadopoulos shared this information with the campaign."
Another unexplained episode involved Papadopoulos' communications with Sergei Millian, a businessman from Belarus, a former Soviet republic. Millian is a naturalized U.S. citizen but he "remained out of the country since the inception of our investigation and declined to meet with members of the Office despite our repeated efforts to obtain an interview."
-- DELETED MESSAGES
Mueller obtained more than 230 orders for communication records, but he was unable to recover some messages. Richard Gates, Trump's deputy campaign chairman, used WhatsApp encryption to share campaign polling data with a Ukrainian business associate who allegedly had Russian intelligence connections. To cover his tracks further, Gates regularly deleted the messages.