They are the victims behind 15 years of court proceedings and legal wrangling: Philip McCloskey, a 69-year-old retired plumber from Taunton and father of six; and 19-year-old Jonathan Rizzo, a college student who wanted to save the world.

Gary Lee Sampson killed them both in a violent spree in the summer of 2001, and for the past 15 years, federal prosecutors have sought the death penalty for the brutal crimes, a process that has led to lengthy appeals.

Relatives of McCloskey and Rizzo have waited for an end to the proceedings, and on Thursday they were back on the witness stand in federal court in Boston. McCloskey’s daughters told the jury how their father enjoyed the beach and the holidays. Rizzo’s younger brothers described how they had looked up to him.

Rizzo’s mother, Mary, told jurors of the car rides they shared, and the songs they sang. She also recounted her heartbreak when she found out her son had been tied to a tree and stabbed. She wanted to see his body but was not allowed. So she reached into a closet and gave a blanket to an investigator, asking him to cover her son’s body.

“I just wanted him covered up, but more than anything I wanted my hands on this blanket, and my love, to be covering him,’’ she told jurors.

Courtroom spectators were in tears, and so were several of the jurors who will determine Sampson’s fate. Sampson, 57, stared straight ahead.

The family members were not allowed to address Sampson directly, but US District Judge Leo T. Sorokin instructed jurors that they could hear the stories of Sampson’s victims, to learn of the harm Sampson had caused their families.

At the end of the emotional testimony Thursday, prosecutors told Sorokin that they have concluded their case, after 11 days of testimony from more than 60 witnesses. Lawyers for Sampson are slated to start calling witnesses on Monday.

Sampson had confessed to killing McCloskey and Rizzo and strangling Robert “Eli’’ Whitney, 58, to death that same week in New Hampshire. He was sentenced there to life in prison for killing Whitney, but federal prosecutors in Massachusetts sought the death penalty in the deaths of Rizzo and McCloskey.

A jury agreed in 2003 to sentence Sampson to death but a federal judge later vacated that decision after finding that one of the jurors lied about her history with law enforcement. Prosecutors are now seeking the death penalty again in a new sentencing trial.

Rizzo’s father, Michael, told reporters outside the courthouse that the families sat through weeks of testimony about crime scene evidence and autopsy results, and wanted to tell jurors the victims’ stories.

“The people get lost in there,’’ he said. “That’s who this is about. That’s all we wanted them to understand, is who they were as people.’’

Charlene McCloskey Murray, Phil McCloskey’s second youngest child, told jurors that her father was a family man. He would pay close attention to her brother, who has autism and needed structure. They would spend weekends together, father and son traveling on errands.

“He just loved being with us, being with his family,’’ she told jurors.

Her older sister, Cheryl McCloskey Shea, recalled holidays. They would decorate the house for Christmas, “even the bathroom.’’ Her father was the type who could walk into a room and make friends with everyone and invite them over for a pool party. “He would go to a barking dog at a fence and make friends,’’ Scott McCloskey told jurors of his father.

Rizzo’s younger brothers, Nicholas, 30, and Elliot, 27, fought back years as they described their family, three young boys traveling to Walt Disney World, or playing Nerf tag in the basement. They played hockey and football and soccer, though Jonathan Rizzo was better at volunteering in the community than athletics, they joked.

Elliot Rizzo recalled the night the family learned his brother had been killed, after a two-day search. “I never believed he wouldn’t come back alive,’’ he said. Nicholas joined the Air Force and worked in a technical rescue unit, a dangerous job that gave him a sense he could help someone in their time of despair, the way his brother had needed help.

“I saw what the loss of Jonathan did to my family,’’ he told jurors.

Michael Rizzo told jurors he had seen his son just before the killings, when they went shopping for dress clothes in Boston. They stopped for dinner, and shared a first glass of wine together, their first “adult conversation.’’

“He was very confident, easy smile … he was very relaxed and easygoing, I guess full of promise,’’ Rizzo said. “It was a good conversation about the future. I told him I was proud of him.’’

Milton J. Valencia can be reached at milton.valencia@globe.com. Follow him on Twitter @miltonvalencia.