(CNN) For a time, Dakota Theriot lived with his girlfriend Summer Ernest and her family in Livingston Parish, Louisiana, a short trip from his home outside of Baton Rouge.

The housing arrangement did not last long. After at least a couple weeks, he was asked to leave the residence and not return, authorities said.

But the 21-year-old returned on Saturday morning wielding a firearm. He fatally shot Ernest, her younger brother Tanner and her father Billy, police said.

Theriot then took Billy Ernest's pickup truck, drove to his family's home in nearby Ascension Parish and killed his own parents, Elizabeth and Keith Theriot, according to Louisiana law enforcement officials.

Keith Theriot identified his son as the shooter before dying, authorities said.

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