Jennifer Carrigan and Steven Daniel Furtado, both 18, were found dead Sunday by Carrigan's mother at a house in Chester.
Deputies arrested 18-year-old Reyes Carillo in the deaths of the two teens, said Plumas County Sheriff's Patrol Commander Gerry Hendrick. He said Carillo previously dated Carrigan.
Furtado was set to graduate on June 6 from Willows High School.
Two rural towns were reeling Monday after the discovery. Chester has a population of about 2,000, and there are about 6,000 people living in the Glenn County town of Willows.
When told of his sister's death, 20-year-old Billy Carrigan headed home from Berkeley.
Shortly before 7 p.m. Sunday, just before he reached Chester, Billy Carrigan crashed his Toyota Tacoma pickup truck into a stand of pine trees along Highway 36.
A spokeswoman at Enloe Medical Center in Chico said he was in critical condition Monday.
It was not immediately clear how Jennifer Carrigan, a senior at Chester High School, and her prom date died.
Autopsies were scheduled for Tuesday at the Washoe County Medical Examiner's Office in Nevada.
Police are calling the case a homicide.
The Glenn County Sheriff's Department said Furtado had traveled from Willows, where he was a high school senior, to Chester to take Carrigan to her senior prom.
Officials delivered the news of Furtado's death to his family on Sunday evening as relatives were in the process of having a Mother's Day celebration at home.
Counseling was set to be offered to Willows High School students in the wake of Furtado's death.
