President Donald Trump has said the FBI doesn't want to investigate Christine Blasey Ford's assertion that Supreme Court nominee Brett Kavanaugh assaulted her, and that it's "not what they do."
In fact, the FBI could certainly investigate Ford's claim, but only if the White House asks the bureau to do so. She has no authority to request it. Neither does the Senate.
When the FBI conducts a background investigation of a presidential nominee, it vacuums up all kinds of information about the nominee, including claims from people interviewed by agents, and dumps it into the file. It does not, however, investigate whether or not derogatory information is true — unless it's asked to follow up by the White House. Several current and former Justice Department and FBI officials say this has always been the practice, and there is actually a longstanding formal memorandum of understanding between DOJ and the White House that specifies these limits.
The Senate cannot ask the FBI to investigate Ford's allegations that Kavanaugh assaulted her at a high school party more than 30 years ago, because Kavanaugh is the president's nominee, not the Senate's.
Here's another way to think about it. In doing background investigations, the FBI is acting as an agent of the White House. That's a separate role from its responsibility to investigate crimes. The Senate can always ask the FBI to investigate a potential crime that it becomes aware of, but it can't direct the FBI to investigate the background of a presidential nominee.
And in this case, even assuming Ford's allegation to be true, there's no suggestion of a federal crime, quite apart from the statute of limitations issue. So the FBI has no independent authority to open a criminal investigation. Its only role here would be to re-open the Kavanaugh background investigation.
Additionally, some in the FBI, says a source familiar with their thinking, are annoyed by President Trump's statements that the FBI doesn't do this or this "is not really their thing" when there are FBI agents who do nothing but interview sex crime victims.