Authorities in Louisiana said they are searching for an "armed and dangerous" 21-year-old accused of killing his parents and three others in two separate but related shootings Saturday.

Ascension Parish Sheriff Bobby Webre says Dakota Theriot is the "prime suspect" in the deaths of Keith, 50, and Elizabeth Theriot, 50, of Gonzales, his parents.

They were shot in their trailer on Saturday morning. Deputies arrived at the scene and were able to interview one of the victims before both died. Webre said that information led authorities to zero in on the couple's son as a suspect.

Dakota Theriot was being sought on first-degree murder and other charges. He was believed to be driving a stolen 2004 Dodge Ram pickup, gray and silver in color.

The sheriff said three other shooting deaths occurred Saturday in neighboring Livingston Parish, about 70 miles (113 kilometers) west of New Orleans.

"After speaking with their detectives for quite a while we came to realize that these two cases indeed may be related. We feel that they are related," Webre said.

Livingston Parish Sheriff Jason Ard confirmed in a statement that three people were killed in his parish and identified the victims as Billy Ernest, 43; Tanner Ernest, 17; and Summer Ernest, 20. The sheriff's office identified Dakota Theriot as the suspect in those killings.

This undated photo provided by Livingston Parish Sheriff’s Office shows Dakota Theriot, 21. Photo: Livingston Parish Sheriff’s Office via AP

Webre said the Ernests were not relatives of the suspect but appeared to know him.

Crystal DeYoung, Billy Ernest's sister, told The Associated Press that she believes Theriot had just started dating her niece, Summer Ernest.

"My family met him last weekend at a birthday party and didn't get good vibes from him," DeYoung said. She said she wasn't sure how her niece and Theriot met, but that she believed the relationship was relatively new.

"My mom is a good judge of character and she just thought he was not good," DeYoung said of Theriot.

DeYoung said she skipped the birthday party and didn't meet Theriot herself. DeYoung said Summer and Tanner Ernest were two of Billy's three children. He was also raising his wife's children.

DeYoung said Theriot doesn't have a vehicle and she's not sure how he ended up at the Ernest home on Saturday, but after the killings, he took off in her brother's truck.

There were also two young children in the home at the time. DeYoung said a 7-year-old took the baby out of the house and went to a neighbor's.

DeYoung said her brother, niece and nephew were good people.

"They all had very good hearts. They trusted people too much," she said, as she began crying. "They all loved unconditionally."

Charlenne Bordelon lives near the house where the Ernests were killed. She told The Advocate newspaper that two young children from the house ran to her home. They were uninjured and asked for help after the shooting.

Bordelon said Theriot was the older daughter's boyfriend and that he'd recently moved in with the family but she did not know him.

A Facebook page appearing to belong to Dakota Theriot was filled with defensive and sometimes angry posts. He shared someone else's post in June that said "wish i could clear my mind jus for one day" (sic) with a sad face emoji.

In May, he reposted something saying, "If you have a problem with me, tell me. Not everyone else."

He also shared someone else's post that said, "I don't care what people say about me I know who I am and I don't have to prove anything to anyone."

Webre said Dakota had lived with his parents briefly but was asked to leave the residence and not return.

"I would not approach this vehicle. We feel no doubt that Dakota is going to be armed and dangerous, and we need to bring him to justice really quick," Webre said.

Webre said Dakota Theriot had some run-ins with law enforcement in other parishes that he described as misdemeanor-type incidents that did not include violence: "Certainly nothing of the magnitude that we've seen today."

Webre said they believe Dakota Theriot is heading east and that there was no reason to think he was now targeting someone else but warned that because he's armed and dangerous: "Anybody he comes into contact with could be a target."