Trump provides written responses to Mueller questions

In 2004, President George W. Bush was interviewed by special counsel Patrick Fitzgerald's office during an investigation into the leaked identity of a covert CIA officer. In 1998, President Bill Clinton testified before a federal grand jury in independent counsel Ken Starr's Whitewater investigation.

"It's very extraordinary if this were a regular case, but it's not every day that you have an investigation that touches upon the White House," said Solomon Wisenberg, a Washington lawyer who was part of Starr's team and conducted the grand jury questioning of Clinton.

Mueller could theoretically still try to subpoena the president if he feels the answers are not satisfactory.

But Justice Department leaders, including acting Attorney General Matthew Whitaker - who now oversees the investigation and has spoken pejoratively of it in the past - would have to sign off on such a move, and it's far from clear that they would. It's also not clear that Mueller's team would prevail if a subpoena fight reached the Supreme Court.

"Mueller certainly could have forced the issue and issued a subpoena, but I think he wants to present a record of having bent over backwards to be fair," Wisenberg said.

The Supreme Court has never directly ruled on whether a president can be subpoenaed to testify in a criminal case. Clinton was subpoenaed to appear before the Whitewater grand jury, but investigators withdrew the subpoena after he agreed to appear voluntarily.

Other cases involving Presidents Richard Nixon and Clinton have presented similar issues for the justices that could be instructive now.

In 1974, for instance, the court ruled that Nixon could be ordered to turn over subpoenaed recordings, a decision that hastened his resignation. The court in 1998 said Clinton could be questioned under oath in a sexual harassment lawsuit brought by Paula Jones.

AP