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Accused killer Dakota Theriot arrives to the Ascension Parish Courthouse Annex escorted by Ascension Parish Sheriff's Office Detective LaTonya Prejean, Friday, Feb. 1, 2019, in Gonzales, La.

Livingston Parish prosecutors said Monday they would seek the execution of a man accused of killing five people, including his parents, in January — aware that Dakota Theriot might suffer from mental health problems but finding nothing in the investigators' files to steer them away from most-severe of penalties.

Theriot pleaded not guilty Monday to three counts of first-degree murder in a Livingston Parish court. He is accused of killing 20-year-old Summer Ernest, whom authorities and her family members have described as Theriot's girlfriend, as well as her father, 43-year-old Billy Ernest, and a brother, 17-year-old Tanner Ernest. 

"You had three completely innocent victims. By all accounts, no justification or mitigating reasons why this took place," Livingston Parish District Attorney Scott Perrilloux said. 

Death penalty cases have become increasingly rare nationwide, and Perrilloux had previously said he was giving this case an especially hard look, due to reports that Theriot could suffer from a mental illness.

"Nothing in any records or information we received so far convinced us not to seek the death penalty," he said Monday.

The Ernest family supported the decision, he said.

Investigators say Theriot also killed his parents in Ascension Parish, where a grand jury on Monday indicted him on two counts of first-degree murder in the shooting deaths of 50-year-old Elizabeth Theriot and 50-year-old Keith Theriot.

Dakota Theriot will appear in Ascension Parish court at a later date for arraignment, according to the Ascension District Attorney's Office.

In the Livingston Parish cases, Perrilloux said he expects Theriot's attorneys to pursue an insanity defense. In pleading insanity, a defendant claims that, due to a mental illness or defect, he could not distinguish right from wrong at the time of the crime. 

Theriot was represented Monday by Elliott Brown, an attorney with the Louisiana Capital Assistance Center, a New Orleans-based nonprofit that defends people charged with capital crimes.

Brown declined to comment.

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The hearing Monday was held before 21st Judicial District Judge Robert Morrison III, but since this is now a capital case, it was randomly re-allotted.

Per local rules, an executive assistant for the court took the stand and rolled a small bingo roller containing balls associated with each division in the district.

The roller produced an "E," meaning the case is assigned to 21st Judicial District Judge Brenda Bedsole Ricks. 

Authorities have said Theriot was staying with the Ernests in a trailer north of Walker for few weeks when, on the morning of Jan. 26, he shot each of them in the head. Then he stole Billy Ernest's truck and drove to Ascension Parish, where he allegedly shot his parents, Elizabeth and Keith Theriot. 

Theriot fled to Virginia and was arrested the following day outside his grandmother's house, authorities said. 

Law enforcement officials have said Theriot confessed to all five killings but did not provide a motive.

Prior to the shooting, Theriot had had repeated run-ins with the police and involuntary mental health holds, law enforcement records show. On different occasions, he threatened to burn down the family house with his parents inside, hallucinated and beat his ex-wife, who told Kenner Police that Theriot was schizophrenic.

Theriot is being held at the Louisiana State Penitentiary at Angola pending trial. 

His next court appearance in Livingston Parish is scheduled for May 9.

Follow Caroline Grueskin on Twitter, @cgrueskin.