UPDATE: Police have arrested two men and a 17-year-old girl in the death of three adults in a Fullerton house, authorities announced Sunday afternoon. They said state law prevents them from identifying the girl, but Katlynn Goodwill Yost, the daughter of the dead couple, knew the suspects. The victims were identified as Christopher and Jennifer Yost, Katlynn’s parents, and Arthur William Boucher. Arrested were Josh Acosta, 21, of Fort Irwin, and Frank Felix, 25, of Sun Valley. More to come.

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FULLERTON – A young girl’s call to 911 on Saturday morning was the first sign of trouble at the modest Fullerton home on Gilbert Street.

She told the operator that her parents had died.

When officers arrived at 401 S. Gilbert, they found three people – two men and a woman – dead inside. Two girls were inside, too, though unharmed, Fullerton police Sgt. Jon Radus said in a statement.

Now police are trying to piece together what happened and are investigating the deaths as a triple homicide after ruling out a murder-suicide late Saturday.

All of the bodies had obvious signs of trauma, Radus said, but he did not know yet what caused it.

“Any time someone loses their life, it’s a tragedy,” Radus told The Associated Press. “It’s even more of a tragedy when children are involved.”

Saturday night, the Fullerton police asked for the public’s help in finding a third girl – 17-year-old Katlynn Goodwill Yost – who also lives at the house. Investigators haven’t been able to find her after searching all day, Radus said.

She is neither a victim nor a suspect at this point, Radus said, describing Yost as a “missing/at-risk person.”

Radus said she may be a family member of one or a few of the victims, but he couldn’t confirm it because the coroner has not yet identified the victims.

“We are concerned for her safety and we are asking the public’s help in locating her,” Radus said.

Around 4 p.m., Fullerton detectives, armed with a search warrant, finally went inside the home to begin examining the bodies and collecting evidence.

“This is obviously a tragic event,” Radus said. “The Fullerton Police Department has committed all its resources to identify who is responsible for committing this act.”

The crime scene drew several neighbors, who gathered behind the yellow tape. Police set up a tarp in front of the small beige house, which had a “Welcome” sign above its front door.

Neighbor Donna Trice, 45, said she had known the family for about three years, and it had a husband, wife and three children, one 17 and the others 7 and 8 or 9.

“It’s sad for the children; they’re now without parents,” she said. “The two little girls found the bodies, and that tears me up.”

Virginia Wheeler, 67, stood behind the tape, hoping that the woman dead inside was not her friend of 18 years.

She said her heart goes out to the woman’s daughters. Wheeler said the woman’s husband was a really good dad who loved playing with the girls. She added that the man was a mechanic who had fixed her car on several occasions.

“Oh, man. I’m in shock,” she said, softly crying.

Trice said she last saw the couple about three weeks ago at a local 7-Eleven. She added that some neighbors reported hearing gunshots Friday night.

Just before 3 p.m., a woman showed up, ducked under the crime scene tape and began crying as she talked with an officer, who put his arm around her. After a few minutes, she got into a Fullerton patrol car and was driven away.

Staff writer Tomoya Shimura contributed to this report.