"For someone to do that, someone so young, someone connected to his victims, the fact that the family took him in and this is the end result, it's kind of jarring for all of us," said Lori Steele, spokeswoman for the Livingston Parish Sheriff's Office.
Authorities said they do not know why Theriot, who had been arrested for minor drug possession but has no history of violent crimes, resorted to such violence. They said his parents kicked him out of their trailer a couple of weeks ago, but they do not know why or whether he had been estranged from his family.
Investigators also have not said what specific type of weapon Theriot used or how he obtained it.
The rampage began shortly before 9 a.m. Saturday, when authorities in Livingston Parish, east of Baton Rouge, received a 911 call about three victims who had been shot, Livingston Parish Sheriff Jason Ard said. Killed were Summer Ernest, 20; her father, Billy Ernest, 43; and her younger brother, Tanner Ernest, 17.
Ard said there were two children inside the home, ages 7 and 1. They were not harmed.
Theriot then stole Billy Ernest's 2004 Dodge pickup truck and drove to Ascension Parish, about 30 miles away, and shot his parents, Elizabeth and Keith Theriot, both 50, inside their trailer, authorities said.
Before he died, Keith Theriot told officers that his son had shot him, Ascension Parish Sheriff Bobby Webre told reporters at a news conference Saturday night.
After the shooting, Theriot drove for 16 hours overnight to Warsaw, Virginia, where some of his relatives live, according to the Richmond County Sheriff's Office. Officials in Virginia had been warned of the slayings in Louisiana and that the suspect has family connections in the area.
Richmond County Sheriff Stephan Smith was not available for comment Sunday, but he told The Advocate that Theriot drove to his grandmother's home in Warsaw. The grandmother and other family members had moved to a hotel in case Theriot tried to contact them.
When Theriot arrived early Sunday morning in Billy Ernest's truck, with a gun pointed out the window, deputies were at the house, the Sheriff's Office said in a news release. Theriot later surrendered.
Smith told The Advocate that Theriot seemed tired and made statements about the slayings. He will face five counts of first-degree murder, among other charges.
Authorities had described Summer Ernest as Theriot's girlfriend and said the two had just started dating. But Paul Hays, Summer's cousin, said that wasn't true and that Summer had just gotten out of a long-term relationship with someone else. He said that the two became friends only recently and that the family just met Theriot at a birthday party last weekend.
"To the whole family, he was a stranger," Hays said.
Summer was simply trying to help a friend who had lost a home, he added.
"The whole family is very loving. We're a close Christian family. We're always on the phone with each other, telling each other we love each other. We're a tight family," Hays said, adding later: "We're praying for his family, too."
"We're really torn apart. Still can't believe it's real even though we got national news going into our driveways," Hays said. "This is really happening. We're never really going to see them again."
A GoFundMe page has been created on behalf of Summer and Tanner Ernest's mother, Rhonda Champagne.
"My babies died for nothing," Champagne told reporters at a family member's home Sunday, according to ABC affiliate WBRZ. "Their daddy died for nothing. For what?"
