A federal jury will begin death penalty deliberations on Friday in the case of a Brooklyn gang leader, Martin Aguilar.

The jury’s sentence could end a span of more than 50 years during which no death sentence was issued from any of New York State’s four federal judicial districts.

In an unusual twist, the defense chose to focus its closing arguments on cold-blooded planning that went into the murder-for-hire for which Aguilar, 34, now faces the death penalty.

Prosecutors gave the murder little more than a passing reference during the government’s closing arguments. Instead, they focused their efforts on convincing the jury that Aguilar would pose a threat to other inmates and correctional officers should the jury opt for a lesser sentence — life in prison.

So the crime that an assistant U.S. attorney, David Bitkower, sought to imprint in the minds of jurors as they prepare to start deliberating was a recent stabbing for which Aguilar has not been charged. In the law library of a prison, Aguilar stabbed a handcuffed inmate in the neck with a shiv made of a pen and piece of metal, Mr. Bitkower said.

This latest crime happened after a day of jury selection, only hours after the jurors had seen Aguilar in court, Mr. Bitkower said.

“It’s because of the defendant’s future dangerousness to other inmates and correctional officers that the death penalty is appropriate in this case,” Mr. Bitkower told the jury.

Mr. Bitkower repeatedly called the death penalty an “appropriate sentence.” His call for execution never went far beyond that.

Aguilar took notes throughout much of yesterday’s proceedings. He kept his eyes on the pad of paper in front of him even as prosecutors requested his death.

Aguilar grew up in Sunset Park, Brooklyn, and was a member of the Bloods and Latin Kings street gangs. His first noteworthy act of violence happened in 1991, when he stabbed a man to death with a screwdriver in a schoolyard. Although acquitted of murder in state court, Aguilar was convicted and sent to prison for threatening to shoot a 14-year-old eyewitness to that murder. From that point on, Aguilar’s life, according to prosecutors, has been a series of stabbings, shootings, and armed robberies.

“We know that he’s killed at least two people,” another assistant U.S. attorney, Todd Harrison, said yesterday, although he suggested that number could be higher.

Yesterday morning, Aguilar briefly addressed the jury for the first time in his trial. Reading hurriedly from a prepared statement, Aguilar told the jury in voice thick with emotion that he had spent much of his life “prepared to die in the street without a thought” because “that’s how gangsters are supposed to do it.”

Aguilar apologized to his mother, Elizabeth Fernandez, who sits each day of trial in the front row of the courtroom of Judge Raymond Dearie of U.S. District Court in Brooklyn. Aguilar expressed particular remorse that his trial prompted his mother to take the stand to plead for his life.

Aguilar said: “I can only hope and pray that one day I will be forgiven by anyone, because in all honesty, I don’t think I can ever forgive myself.”

A defense lawyer, Carl Herman, directed the jury’s attention to the murder that is at the center of the trial. He suggested that Aguilar did not deserve all the blame for the murder of a low-level drug dealer, Jose Fernandez. Mr. Herman reminded the jury to apportion some of the blame to two other men and the victim’s wife, who have been charged for the murder.

Aguilar was the triggerman in that murder, which stemmed from a love triangle involving the victim’s wife and another drug dealer. The defense had called the wife, Quincy Martinez, to testify. It was Ms. Martinez who gave the go-ahead for the murder, Mr. Herman said. He told the jury that the defense had called her forward as a witness so that each juror “could see the face of evil.”