A man accused of killing his girlfriend, her family and his parents in Louisiana has pleaded not guilty. Prosecutors say they're seeking the death penalty.

News outlets report 21-year-old Dakota Theriot entered the pleas Monday to three counts of first-degree murder in the deaths of his girlfriend, Summer Ernest; her father, Billy Ernest; and her brother, Tanner Ernest.

Authorities say Theriot shot the Ernests in Livingston Parish, then his parents, Elizabeth and Keith Theriot, in Ascension Parish. He was arrested in Virginia . The five slayings happened in January.

Livingston Parish District Attorney Scott Perrilloux says he considered reports that Theriot is mentally ill, but ultimately nothing convinced prosecutors not to seek his execution. Perrilloux expects an insanity defense.

Theriot hasn't been indicted yet in Ascension Parish. His lawyer declined to comment to The Advocate .

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Information from: The Advocate, http://theadvocate.com