HOULTON, Maine — Jury selection continued for a third day Friday at Aroostook County Superior Court in the Thayne Ormsby triple-murder trial.

Of the 14 potential jurors questioned Friday morning, eight were excused. The addition of six more people to the pool of potential jurors brought to 20 the total number of people deemed impartial and able to serve.

Deputy Attorney General William Stokes, who is prosecuting the case, said during a break that there must be at least 36 or 37 people from which 12 jurors and two or three alternates can be chosen. The prosecution and the defense team each have 10 preemptive strikes they may use to remove jurors from the pool and one each for the alternates.

When Superior Court Justice E. Allen Hunter broke for lunch Friday, 37 potential jurors remained to be questioned out of the 118 who appeared Wednesday for jury duty.

Ormsby has changed his plea to not guilty by reason of insanity, a court clerk and the prosecution confirmed Friday morning, despite his having been ruled competent to stand trial. The date of the change of plea was not immediately available.

The defense has repeatedly sought to have jurors removed when they indicated an inability to judge how mental illness might affect behavior.

Opening arguments and testimony are scheduled to start Monday afternoon.

Because of his insanity plea, Ormsby will be in tried in two phases. In the first and longest phase, the jury will be asked to find if he is guilty or not guilty of the charges on which he has been indicted. If he is found guilty, the jury will hear evidence as to his state of mind at the time of the crime. Jurors then will be asked to determine whether Ormsby was criminally responsible for his actions.

If the jury found he was insane when the crimes were committed, Ormsby would not be sent to prison but to the Riverview Psychiatric Center in Augusta for an undetermined amount of time.

At the end of the day Thursday, many of the 118-member jury pool remained to be questioned individually about their knowledge of the trial and relationships to potential witnesses, according to a previously published report. The judge and attorneys also asked potential jurors about their experiences with the law or inherent biases that might prevent them from judging the case fairly.

Ormsby, 21, initially pleaded not guilty in July 2010 to three counts of murder and an arson charge in connection with the deaths of Jeffrey Ryan, 55, Ryan’s son Jesse, 10, and Ryan family friend Jason Dehahn, 30, all of Amity, on June 22, 2010. They were found dead June 22, 2010, about 27 hours after the killings at the Ryans’ home on U.S. Route 1, according to police.

Police said the victims were stabbed to death with a combat knife that Ormsby reportedly always carried with him before he took Ryan’s pickup truck and burned it with the aid of an accomplice. State police divers have found the knife, officials said.

Police said the defendant confessed to killing Jeffrey Ryan because he believed Ryan was a drug dealer. Ryan’s family has denied the claim, and a criminal background check on Jeffrey Ryan revealed no history of drug-related offenses.

Ormsby’s attorneys filed paperwork seeking to suppress statements that he made to police in June and July of 2010, claiming that his Miranda rights were violated. Superior Court Justice E. Allen Hunter rejected the motion in December 2011.

Jury selection was scheduled to resume at 1 p.m. Friday after the lunch break.