Police in Louisiana have launched a manhunt for an “armed and dangerous” 21-year-old accused of killing his parents and three others in two separate shootings.

Dakota Theriot was the “prime suspect” in the deaths of his parents, Keith, 50, and Elizabeth Theriot, 50, who lived in the town of Gonzales about 70 miles (113km) west of New Orleans.

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. Ascension sheriff Bobby Webre said that information led authorities to zero in on the couple’s son as a suspect.

Dakota Theriot was being sought on charges including first-degree murder. He was believed to be driving a stolen 2004 Dodge Ram pickup.

Investigators at the house of Keith and Elizabeth Theriot in Ascension parish, Louisiana. Photograph: Advocate staff photo by HILARY SCHEINUK/AP

The sheriff said three other people were shot dead on Saturday in neighbouring Livingston parish on Saturday with Theriot as the prime suspect.

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

Livingston 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.

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

Crystal DeYoung, Billy Ernest’s sister, told Associated Press that she believed 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 was not 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, adding that she had skipped the party and did not meet Theriot. DeYoung said Summer and Tanner Ernest were two of Billy’s three children. She believed Theriot had stolen Billy Ernest’s truck.

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

“They all had very good hearts. They trusted people too much,” she said. “They all loved unconditionally.”

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

“I would not approach this vehicle. We feel no doubt that Dakota is going to be armed and dangerous,” Webre said.

