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James Mulvaney, 52, son Jason, 12, and daughter Jennifer, 7, were declared dead at the scene at 5:54 p.m.
Early Thursday morning, the coroner removed the bodies from Mulvaney's apartment in Thousand Oaks.
Authorities say the children's father, James Mulvaney, killed 12-year-old Jason and 7-year-old Jennifer, both stabbed to death in their bedrooms, before he overdosed on prescription drugs. The killings happened some time after Tuesday afternoon, when the mother dropped off the kids for an overnight-custody visit.
Investigators combed the crime scene for more than 12 hours, collecting evidence as they try to piece together what happened.
Police described the scene inside the apartment as so disturbing that even the paramedics were shaken by what they saw.
"It was a gruesome scene, very disturbing. We had chaplains here, not only for the very distraught ex-wife, but also for the first responders who had to deal with this scene when they arrived," said Ventura County Sheriff's Detective Eric Buschow.
Neighbors said they heard the distraught mother's screams.
"I've never seen such raw, human, terrifying emotion before in my life," said neighbor Bonnie Shapiro. "
Shapiro was the first person to see the distraught mother, seconds after she found the bodies inside the apartment.
"I went over to her and they had me talk to 911 for her, because they couldn't understand her," said Shapiro. "It was pretty traumatic."
"I don't really understand how you kill your own, somebody that is your own blood," said neighbor Angelica Kope.
Thursday, at nearby Los Cerritos Middle School, where Jason attended, crisis counselors were helping students understand the senseless killing.
"The school is pretty much numb right now," said Dr. Jeff Davis, Los Cerritos Middle School. "Very somber mood. Everybody's really bothered by this, everybody's really upset by this. This kind of stuff doesn't happen here. This is the type of thing that Thousand Oaks is not used to and does not ever want to get used to."
Jennifer attended Lang Ranch Elementary School. Crisis counselors were on hand there Thursday.
Parents who knew the second-grader were stunned.
"She was a very loving girl," said parent Lori Gadsby. "Very sweet. I think everybody is in shock. We can't believe that happened."
"We're in Thousand Oaks, you know? People move here because it's the safest city in L.A.," said Bonnie Shapiro. "It's just strange. It makes me want to not live right here anymore. Even if I moved so I don't have to come home every day and drive by that and re-live the mom. I don't think I'll ever wash that out of my head."
Eyewitness News Reporter Amy Powell contributed to this report.