Why Tsarnaev prosecutors want to keep Catholic nun off the stand (+video)

As Dzhokhar Tsarnaev's lawyers wrap up their arguments to spare him the death penalty, prosecutors are attempting to block testimony from their final witness: a prominent death penalty critic and Roman Catholic nun.

As the team of lawyers defending Boston Marathon bomber Dzhokhar Tsarnaev prepare to wrap up their case, they've run into opposition from prosecutors over their last potential witness.

The witness is Helen Prejean, a Roman Catholic nun and prominent death penalty critic whose autobiography served as the basis for the Susan Sarandon and Sean Penn film "Dead Man Walking" about a death row inmate in Louisiana. She was in court on Thursday but is yet to take the stand as the government signaled its intention to file a motion to exclude her testimony, according to WGBH.

The defense wants Sister Prejean to testify about what she has described as the relentless "torture" of death row, but prosecutors may be wary of her influence in Boston, a heavily Roman Catholic community, according to legal experts.

The prosecution might fear that some jurors "may admire her and be swayed by her mere presence as a defense witness," says Daniel Medwed, a professor at Northeastern University School of Law in Boston who has been following the case, in an e-mail to the Monitor.

"In the eyes of the prosecution, this risk might be particularly high because of the large Catholic community in Boston and the likely presence of Catholic jurors who may know about Sister Helen's reputation," Professor Medwed adds.

It's unclear on what grounds the government is challenging her testimony, but Medwed says that relevance would be the best argument against her.

"Why is her testimony actually relevant to the mitigation case here?" asks Medwed. "The prosecution will probably contend that the value/relevance of her testimony to the case at hand may be quite minimal in comparison to the risk that her mere presence might unduly affect the jury out of respect and affinity for her, and not the merits of the specific mitigation defense in the case at hand."

After lengthy discussions in the judge's chambers Thursday, Judge George O'Toole ended the day's testimony without Prejean taking the stand. If she does ultimately testify, she is expected to be the last defense witness when they wrap up their case Monday. Prosecutors would then have a short rebuttal before both sides issue closing statements. 

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The defense has called 40 witnesses so far during two weeks of testimony, including several of the defendant's family members. Once testimony is complete, the jury will convene to decide if Mr. Tsarnaev should be sentenced to death or to life in prison. (Testimony this week has covered the conditions Tsarnaev would likely face at the supermax ADX prison facility in Florence, Colo.)

Medwed described calling the nun to testify a "brilliant strategy" by the defense because her presence would "add gravitas" to their case, and also because the government would be hard-pressed to cross-examine her aggressively.

In theory, Prejean's testimony shouldn't be given any extra weight over other witnesses, says Rosanna Cavallaro, a professor at Suffolk University Law School in Boston, in an e-mail to the Monitor.

"The appropriate question for the jury is whether mitigating circumstances [in the case] outweigh aggravating circumstances," says Professor Cavallaro.

"That a particular clergywoman believes death is an inappropriate sentence has not in the past been recognized as a mitigating circumstance," she adds, "but the law is also clear that the court cannot foreclose consideration of any mitigating circumstance. So the safer course would be to allow her to testify."

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