Can the FBI take custody of a defendant in a state trial if they have information the FBI needs (possibly national security related)?
<p>The premise of the question is based on <a href="https://www.imdb.com/title/tt0775648/" rel="nofollow noreferrer">Law and Order episode <em>America, Inc.</em></a> The situation is as follows: a person is facing serious state charges (murder), a trial is underway and jeopardy has attached. Then the FBI approaches the DA to say that they need the defendant's help and that the lives of American servicemembers may be at risk. In the episode, it's made to look as though there are two options to make the defendant available to the FBI: a plea bargain (requiring the DA's Office and the defendant's cooperation) or a vacation of the charges (meaning the defendant walks because jeopardy has already attached; also requiring the DA's Office to cooperate).</p>
<p>I'm wondering if there's a third option whereby the FBI can take custody of the defendant (temporarily) without the DA's Office's cooperation. Under what circumstances, if any, does that option exist and how does it affect the state case against the defendant?</p>
<p>I realize it may be possible to request a continuance in the state case, possibly requiring cooperation from parties involved in the state case. I am interested in answers where the FBI (or more broadly the federal government) can force taking custody of the defendant without cooperation from the state (other than compliance with federal law).</p>

<p>Basically yes. Assuming there’s a grand jury investigation (which the FBI needs if it wants to issue subpoenas), the government can get a writ of habeas corpus <em>ad testificandum</em>. This orders the warden to deliver the prisoner to the grand jury to testify. Interrupting a jury trial is a pretty big request, so the federal court might not issue the writ unless there’s a good reason. But ultimately, a state cannot refuse to obey a federal writ of habeas corpus.</p>

united-states fbi tenth-amendment