The death-penalty trial of admitted serial killer Gary Lee Sampson began Sept. 14 in federal court in Boston with the process of selecting a jury. Opening statements could occur as soon as next week. Here’s an explainer on the most high-stakes trial in Massachusetts.

What is the case about?

Sampson, 57, a drifter from Abington, said he carjacked and killed 19-year-old Jonathan Rizzo and Philip McCloskey, 69, in separate incidents during a violent spree in July 2001. During the same week, he also killed Robert “Eli’’ Whitney, 58, in New Hampshire. Although he admitted to the crimes, a jury must decide whether he should be sentenced to death or life in prison without parole.

Why does he face the death penalty?

Massachusetts does not have a death penalty, but the United States court system does for certain crimes, such as carjacking resulting in death. Federal prosecutors chose in 2001 to seek capital punishment based on the severity of the crimes.

Why is the case happening now?

Sampson pleaded guilty in 2003 and a federal jury condemned him to death that same year. After a lengthy appeals process, a federal judge vacated that jury’s decision, however, after finding that one of the jurors lied during a screening process. An appeals court upheld the decision. Federal prosecutors agreed to seek the death penalty in a new trial.

Why has it taken more than a month

to pick a jury?

To be picked for death-penalty cases, potential jurors must be willing to hand out the death penalty if they find it is warranted; anyone who is inherently against capital punishment is excluded. More than 600 jurors filled out a questionnaire to help screen their backgrounds. US District Judge Leo T. Sorokin is interviewing those who were not excluded, in what is known as a voir dire process. The judge hopes to find about 70 qualified jurors before letting lawyers and prosecutors pick the final jury.

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