If special counsel Robert Mueller really wants to interview President Donald Trump for his wide-ranging investigation, he's got one way to do it: with a subpoena.
"It's a court order to show up at a certain time and place. At that point, the free opportunity for coming in is over," said Alan Morrison, associate dean at George Washington University Law School.
The Washington Post reports Mueller raised the prospect of subpoenaing Trump after Trump's lawyers told him in March the president had no obligation to sit down for an interview.
Trump wouldn't be the first president to be subpoenaed. President Richard Nixon famously received a subpoena in the Watergate investigation to turn over tapes of conversations recorded in the Oval Office. President Bill Clinton was subpoenaed to testify in a sexual harassment lawsuit Paula Jones filed.
But subpoenaing a president can be tricky, thanks to something called executive privilege, a defense the president can use to avoid answering questions or appearing in court.
"There's two kinds of privileges; one is the subject of what the privilege is about, that is to the extent that they're talking about matters that are within the president's powers," Morrison explained. "He may say, 'No, matters relating to personnel are exclusively mine, and I claim a substantive privilege with that.'
"The second privilege, which is different, is the burden on the president taking him away from his other duties."
The idea of executive privilege goes back to President Thomas Jefferson, who argued he couldn't be compelled to appear in court for the trial of Aaron Burr because leaving Washington to do so would leave the country without an executive branch.
Both Nixon and Clinton also claimed they couldn't comply with their respective subpoenas, but the Supreme Court ruled against them, arguing the judicial process can overrule executive privilege in some cases.
"If I understand special counsel Mueller's investigation, of course it includes much of what happened before Donald Trump became President Trump; so in that situation, he has no legitimate privilege, executive privilege," Morrison said.
If Trump is subpoenaed, his legal team will have to negotiate with Mueller to determine when, where and how the interview will take place, or they could try to object to the subpoena or certain parts of it.
"It could be that you have to come, but you don't have to come for more than this number of hours, these are the subjects you have to talk about; and sometimes you'll see them where they will object to the questions, and they'll have to go to the judge to see whether the judge will direct that the witness answer the questions," Morrison said.
Trump could also agree to show up for an interview but refuse to answer questions by pleading the Fifth Amendment, which gives Americans the right not to answer questions that could lead to their incrimination.