By Scott Malone

BOSTON (Reuters) – A judge told prosecutors on Wednesday that they may not mention that the accused Boston Marathon bomber is a naturalized citizen as a factor in determining whether he should be executed if convicted of the deadly attack.

Dzhokhar Tsarnaev is the surviving member of a pair of ethnic Chechen brothers who immigrated to the United States about a decade ago whom federal prosecutors contend killed three people and injured more than 260 in the largest mass-casualty attack on U.S. soil since Sept. 11, 2001. Tsarnaev’s 26-year-old brother, Tamerlan, died after a gunbattle with police three days after the April 15, 2013 bombing.

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U.S. District Judge George O’Toole blocked a bid by prosecutors to cite the fact that Tsarnaev took an oath of loyalty to the United States when he became a citizen as an aggravating factor in deciding on the death penalty if he is convicted.

The judge noted that a native-born citizen could not face the same claim because they do not take such an oath.

“It’s unduly prejudicial and I will strike,” O’Toole said.

O’Toole also denied a request by lawyers for Dzhokhar Tsarnaev, 20, to allow them to meet jointly with their client and his sisters without having federal agents present.

Defense attorneys had argued that understanding accused Tsarnaev’s relations with his family would be important to making a case that he does not deserve execution if convicted, saying the presence of a federal officers prevented normal conversations.

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O’Toole instead approved a proposal by prosecutors to have a Federal Bureau of Investigation agent from outside Boston, who is not directly involved in the investigation to monitor the meetings at the prison west of Boston where Tsarnaev is being held.

Tsarnaev has pleaded not guilty to the charges. His trial scheduled to begin in November.

Defense attorneys also face a Wednesday deadline to make their case to move the trial outside of Boston.

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U.S. laws generally require a person accused of a crime to stand trial in the district where it was committed. Defendants can seek to have proceedings moved to another location where potential jurors may have been less influenced by pretrial publicity.

Another judge at U.S. District Court in Boston last month rejected a bid to move out of state the upcoming trials of three friends of Tsarnaev accused of interfering with the investigation by taking a laptop and backpack from his dormitory room.

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The case is USA v. Dzhokhar Tsarnaev, U.S. District Court for the District of Massachusetts No. 13-10200

(Reporting by Scott Malone; Editing by Grant McCool)