Robert Mueller likely thought Don Jr. was guilty — here's why that actually made it hard to investigate Trump

Special counsel Robert Mueller has completed his investigation of ties between Russia and President Donald Trump's campaign, turned over his findings to Congress, and stepped down from his post at the Justice Department.

His findings were incredibly damning for the president and his allies, finding evidence that the campaign eagerly accepted Russian help, if not a full-blown conspiracy, and outlining ten potential episodes where Trump obstructed justice. But Mueller's conclusions are by no means the end-all of everything that happened. Mueller himself acknowledged in his report that Trump's lack of cooperation probably prevented him from finding a lot of information.

In fact, as former U.S. Attorney Barbara McQuade wrote in Just Security, Mueller might paradoxically have found it harder to investigate the president because he believed his son was guilty of serious offenses.

"Typically, immunity is offered only to witnesses whose own culpability is less than the individuals against whom they are testifying. Mueller appears to have concluded that the witnesses who invoked the privilege were themselves too culpable to offer them immunity," wrote McQuade. "A fair inference from reading the report is that Donald Trump, Jr., likely invoked his Fifth Amendment right against self-incrimination to refuse to testify before the grand jury."

Trump Jr. was a massive focus of Mueller's investigation owing to the infamous Trump Tower meeting in which he met with a Russian lawyer promising dirt on Hillary Clinton's campaign during the 2016 election.

"While Trump, Jr., to date has not been charged with a crime, he may have had valid concerns during the investigation that his answers would incriminate himself," wrote McQuade. "Mueller did not grant Trump, Jr., immunity, placing him in that category as someone whose own culpability made him a poor candidate for immunity. Trump, Jr.'s refusal to answer questions prevented Mueller from fully exploring the facts about the Trump Tower meeting. While any witness has a constitutional right to invoke his Fifth Amendment privilege, Mueller states that the privilege contributed to his inability to conduct a complete investigation."

Ironically, then, if the president did in fact commit a crime, one of the key reasons the special counsel could not determine this may have been that his son was involved as well!