The 17-year-old named Katlynn Goodwill Yost went missing after the three were found dead.
Two men were arrested and a 17-year-old girl was detained Sunday on suspicion of killing three people inside a Southern California home over the weekend, police said.
Katlynn Yost was located, but police said state law prevents them from saying whether she was the arrested teenager.
The 17-year-old teen was booked into Orange County Juvenile Hall, while Joshua Charles Acosta, 21, of Fort Irwin, and Frank Sato Felix, 25, of Sun Valley, were booked into Orange County Jail.
First responders who were dispatched to the home in Fullerton, a suburb 26 miles southeast of Los Angeles, on Saturday discovered two men and a woman dead.
Radus identified the victims - Christopher Yost and Jennifer Goodwill Yost - as the girls' parents and described the third victim, Arthur William Boucher, 28, as a family friend. Police said the 6-year-old and 9-year-old were unharmed when they arrived, but 17-year-old Katlynn was missing.
"Our investigators were unable to locate her throughout the day, through various investigative means as well as trying to reach her on the phone", says Radus.
The home is situated on a block of single family residences in a mostly middle-class community. "It is our hope these arrests will bring some comfort to the community and bring some peace to the families that were affected by this absolutely unthinkable crime".
On Tuesday, police will turn the case over to the Orange County District Attorney's Office where prosecutors will decide whether or not to file formal charges.
"We don't believe the community is now in danger at this particular time", he said. "They were really good parents too", said Kim Gracier, Boucher's mother, of the Yosts. "We believe that the individuals who committed this crime specifically sought out this residence and the people inside".
The motive is still unknown, and officers have not yet disclosed how the victims died.
She added he was a mechanic who had fixed her cars on a number of occasions, before saying: "Oh man, I'm in shock". They announced on Sunday she had been found.
"He was always standing up for women in his family, me included", his girlfriend Joyce Burchett said through tears.