Did President Donald Trump obstruct justice in the course of the Russia investigation? According to a new letter from Attorney General Bill Barr, Special Counsel Robert Mueller did not answer that question directly, only providing evidence for and against the proposition. But Barr and Deputy Attorney General Rod Rosenstein concluded, reading Mueller’s report, that there is not enough evidence to bring the charge of obstruction against Trump.

But there’s a key problem with this conclusion: Trump was never formally interviewed by Mueller.

Though he answered written questions from the special counsel, Trump steadfastly refused to sit down with him, despite having promised that he would testify in the case under oath. And this is particularly problematic because, as Barr noted in his letter, Trump’s intent with regard to potentially obstructive acts is a key factor when determining whether a crime was committed. How can the investigators come to a conclusion about Trump’s intent without asking him questions and assessing his answers?

Mueller reportedly wanted to interview Trump and tried to get him to agree to it repeatedly, but the investigators were never able to come to an agreement with the president’s lawyers. It’s not clear why Mueller never asked to subpoena Trump — many legal experts believe such an attempt would have been successful.

According to CNN, a source said, “The special counsel’s office deliberated at length with DOJ officials about issuing a subpoena for President Donald Trump to be interviewed, but ultimately the decision was made not to move forward.”

Even after all his criticism of Hillary Clinton and the way the investigation of her emails was conducted, ultimately, she spoke to the FBI about the case — facing the potential for criminal charges if she lied to them. But Trump couldn’t even do that.

The reason why is relatively obvious: His lawyers feared that there was no way Trump could talk to the FBI without lying.

But this shouldn’t be acceptable. In a central national security investigation, any federal employee, let alone the president, should be expected to speak with investigators thoroughly and honestly. They may invoke the Fifth Amendment if they fear self-incrimination, but they should not be allowed to keep their jobs in the government if they do.

The fact that Trump was never forced to answer straightforward questions about what he did and what he knew when he did it will forever hang as a glaring omission in the Russia probe and in Mueller’s final report.