What authority?

Atrios asks the same question I have:

What possible authority does the White House have to try to prevent a former employee from testifying about something? How can they White House "not let her" testify?

Obviously if she doesn't want to testify they can play legal games to try to prevent her from being compelled to testify, but she's said she's willing. How can they stop her? I don't get it.

Ahhhh. State secrets, perhaps? [The state secret being the crimes committed by Bush/Cheney].