Title 18, Section 1512 of the U.S. Code makes it a crime to tamper with a witness, a victim, or an informant. As a result, a lawyer doesn't need a judge to tell her that she can't kill, threaten, intimidate, or "corruptly influence" a witness so as to prevent or alter his testimony. (The law against obstruction of justice can also apply to cases of witness tampering.) But judges do have to be explicit in their proscription of less flagrant practices, like excessive witness "coaching."
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Judges can try to keep this from happening by sequestering the witnesses. That doesn't mean that a bailiff will confiscate their newspapers and lock them in a hotel—but it does mean that the lawyers can't tell them what other witnesses are saying at the trial. (In the Moussaoui case the witnesses were instructed to avoid media accounts of the trial as well.) Either side can request one of these sequestration orders under Rule 615 of the Federal Rules of Evidence, or the judge can issue it at her own discretion. In fact, it's common practice for a judge to sequester the witnesses at the start of a criminal trial.
What if a prosecutor coaches the witness a little bit too much before the trial even starts? Lawyers can start prepping someone for the stand well before the judge has a chance to issue a sequestration order. That means they can present their witness with documentary evidence that they plan to introduce in court, or they can tell him what their other witnesses are going to say under oath. These practices wouldn't necessarily run afoul of Rule 615, nor would they amount to criminal tampering under section 1512. But lawyers are under an ethical obligation to avoid coaching a witness in such a way that would distort his testimony. A prosecutor who violated this code might not face criminal and civil contempt citations (like the ones threatened for the Moussaoui blunder), but she could find herself in front of a state ethics committee.
Martin will have a bit of free time to review those codes and rules; she was placed on administrative leave this afternoon:
In court on Tuesday, U.S. District Judge Leonie Brinkema said that Martin violated federal witness rules when she sent trial transcripts to seven aviation witnesses, coached them on how to deflect defense attacks and lied to defense lawyers to prevent them from interviewing witnesses they wanted to call.
Brinkema warned her that she could face civil or criminal charges and that she appeared to have violated rules of legal ethics.
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On Thursday, Martin, 51, was placed on paid administrative leave from her job as a Transportation Security Administration attorney, where she earned about $120,000 a year.