Ah. Somehow I hadn't caught that this was a securities case, and I thought there might be an element of suspicion that the defendant had something to do with the witness's death. If that's NOT the case, then things could go quite differently.
Securities fraud would, in addition to being investigated by the SEC (tip o' the hat to the poster who just mentioned that), be in federal court, not state court. In federal courts the prosecutor is called a U.S. Attorney or an Assistant U.S. Attorney (those are two different ranks/experience levels of attorneys representing the U.S. federal government), but I think--though am not sure--that there is a *slight* possibility of a prosecuting attorney from the SEC. Note that if this is a big case, there will be more than one lawyer on each side--for the prosecution, a main lawyer and then maybe one or two more junior ones helping/observing the main one; ditto for the defense. Given that it's in federal court, it doesn't matter what state it's in--federal courts all run by the same rules, regardless of where they're located. Federal cases, as compared to state cases, are generally (1) more formal and decorous, and as a result, (2) more boring to watch. So yeah, don't try making a courtroom drama out of this!
As for political pressure influencing the case, that's quite believable; for more details I suggest you read about the recent blowup involving the Bush administration's attempts to pressure/fire/sideline U.S. Attorneys who didn't follow the party line (i.e. who refused pressure from the Bush administration to handle their cases a certain way).
As for the investigation's being ended, keep in mind that in the US you've got essentially two investigation-like phases in any criminal case: (1) the investigation to determine whether there's enough evidence to bring charges, in which case the charges are brought and the judicial machinery starts moving (setting hearing dates etc.); and then (2), during the court phase but before the actual trial, there's the "discovery phase," where the prosecution tries to find every bit of evidence it can to make the charges stick, and the defense tries to come up with everything it can to get out of the charges (e.g. exculpatory witnesses, etc.). A case will be "in discovery" (i.e. in the discovery phase) for a while before the actual trial; that means charges have been filed, the case is on the docket, and some hearings involving only the judge and the lawyers may happen--hearings to determine what evidence can come in, etc.--and this is all before the jury is even chosen.
Generally prosecutors have no reason to speed a trial forward; either they've got all the evidence they need and have no problem handling their other cases until the trial for this one begins, or they don't yet have all the evidence and need more time to look for more evidence. Merely thinking you have a slam-dunk case isn't really a reason to rush ahead. Political pressure IS a reason prosecutors might try to speed things up, so you might want to emphasize that--like, show the prosecutor being pressured or talking about being pressured. Remember that prosecutors in this kind of case are not elected (unlike DA's/district attorneys, or solicitors as some places call them, at the state level), so that kind of political pressure is a non-issue here.
Also, unless this is an unusual kind of securities case, the pressure some district attorneys get from the public (e.g. people flipping out because some rapist still hasn't been caught or some child molester was let out on bail) is unlikely here. The pressure will most likely be some kind of personal or partisan vendetta (whoever's in power trying to ruin the life of the defendant), and pressure will most likely flow as follows: politicos pressure the U.S. attorney for that district; he or she then pressures the assistant U.S. attorney who's actually handling the case.
And also, when a witness dies their deposition testimony will probably be allowed into evidence. That is, the judge will probably let a transcript of what the witness said under oath during a deposition (i.e., during pre-trial questioning that was done in the discovery phase) be read to the jury. There's an exception to the hearsay rule when the witness is dead, though it's not automatic. The judge could perhaps say something to that effect--something like asking the prosecutor, "You've got deposition testimony from this witness, am I right?" (a judge probably wouldn't TELL the prosecutor what to do, i.e. she wouldn't give instructions like "just move to admit the deposition testimony," but she might ask this kind of rhetorical question to make the prosecutor realize that he should, indeed, file a motion [i.e. "move"] to admit the deposition testimony).
Then when the prosecutor answers yes, and says something about filing a motion to admit it, the judge might say something to the effect of, "Right. So, if you need time to prepare your arguments we can adjourn until tomorrow morning and then move forward. We are not going to rearrange this court's entire docket around this unfortunate development." (In other words, the judge refuses to screw up the schedules of every other case that's scheduled to be heard in her court just for this one problem with the prosecutor's case.)