TEMECULA -- Matthew Blixt murdered four people, including his father, then turned the gun on himself in Temecula's worst-ever case of murder-suicide, authorities confirmed Monday, nearly five months after the shootings.

"The investigation determined Matthew Blixt was solely responsible for the homicides," said Riverside County sheriff's homicide Lt. John Schultz.

As a matter of protocol, investigators will now submit their findings to the Riverside County district attorney's office for review.

Jeffrey Blixt, 45; his 34-year-old girlfriend, Naomi Grangroth; her twin 15-year-old daughters, Narissa and Nikita Williams; and Blixt's 17-year-old son, Matthew, all died from gunshot wounds.

Matthew Blixt's younger sister, Danielle, was the only one in the home that day to survive the killings. She was sleeping and woke up to find the bodies, authorities said.

While Grangroth's father and younger sister said Monday they appreciate that investigators finally have released their conclusion, both said they still have questions -- including what Danielle may have known.

"I'll probably never know why he did it," Grangroth's father, Dave Richardson, said by telephone from Washington on Monday. "It would be nice to know if she knows why he did it. That would bring us all much more closure -- to find out why this happened."

Only Jeffrey Blixt lived at the single-story home on Iron Circle in the Temeku Hills area of the city. Matthew Blixt and Danielle lived with their mother in the Desert Hot Springs area, authorities said.

Friends and relatives have said Jeffrey Blixt was having a Sunday barbecue Nov. 11 with the hope of bringing his family together with his girlfriend's family.

Nearly five months after the bodies were found, investigators still can't say why the slayings happened.

No suicide note was found and a forensic examination of Matthew Blixt's computer revealed nothing to help investigators determine a motive, homicide Sgt. Michael Lujan said Monday.

"We have no indication this was planned or thought out," Lujan said.

Toxicological tests indicate Matthew Blixt had no alcohol or drugs in his system, the sergeant said.

Blixt, his father and Grangroth were found dead in the backyard. The teenage girls -- sophomores at Great Oak High in Temecula -- were found inside the home; Nikita died there and Narissa was pronounced dead at an area hospital.

Under Matthew Blixt's body, investigators found a Hi-Point 9mm semiautomatic handgun.

The gun belonged to Jeffrey Blixt and was normally kept unsecured in the master bedroom of the Temecula man's home, Lujan said. No shots other than the ones that killed the five were fired from the gun, the sergeant said.

Matthew Blixt's fingerprints were found on the magazine that holds the bullets in the semiautomatic weapon, Schultz said.

The weapon, bullets and expended casings investigators found were all sent to the California Department of Justice for analysis. That examination has confirmed that all the bullets were fired from the 9mm handgun found under the teen's body, investigators said.

Test results were released to investigators Monday morning, Schultz said.

The delay in getting those results has frustrated family members of Grangroth and her twin daughters.

"It was incumbent upon us to do a slow, steady investigation in this case," Schultz said. Investigators did not want to release their conclusions until they had all the reports back from the Department of Justice.

Authorities say much of the delay stemmed from the state agency being backlogged with other cases and the need for them to prioritize their tests. With no one in custody waiting to go to court and no suspects on the loose, the Temecula investigation became a lower priority, officials have said.

Gunshot residue tests came back positive on Matthew Blixt, Schultz said. Those tests are not an absolute, he said, but do help investigators get some indication of who may have fired a weapon.

Gunshot residue tests were also done on Danielle, the sole survivor that day. No gunshot residue was found on her, Schultz said.

"He definitely fired the weapon or was very close to it when it was fired. She undoubtedly wasn't," Schultz said.

After the shooting, some questioned why it took Danielle about 20 minutes to call 911 and why she did so from a fast-food business some distance from the neighborhood where her father lived.

Danielle was interviewed extensively by investigators, Lujan said.

"There is no evidence that points to her as a participant in the homicides," Lujan said.

"We also have no indication she knew Matthew (Blixt) was going to do this," the sergeant said.

He added that investigators also don't know why Danielle was spared from the gunshots.

Regarding the girl's delay in calling 911, Lujan said she was clearly in shock when interviewed by police.

"People react differently to trauma," he said.

Despite investigators' contentions that Danielle was not involved, Grangroth's father and sister both said Monday they believe she has to at least know something.

"I want to know about Danielle," Aubrey Grangroth, 32, said by telephone from Washington.

"That was my sister and my nieces that were murdered. I feel I have a right to know everything about what happened," Grangroth said.

For months, she has waited to have her questions answered and says those involved in the investigation rarely spoke to her.

"Now I'm irritated that I waited this damn long and I'm still not finding out anything more about what happened," she said.

Richardson, Naomi Grangroth's father, said hearing the conclusion of investigators leads to minimal closure for him.

"It doesn't bring back my daughter or granddaughters," he said.

Richardson said he was just recently getting to know his daughter again.

"That's why this is so hard," he said.

He didn't know he had a daughter until Naomi was 16 years old. He said he was able to attend her high school graduation, but the two went their separate ways as she grew older.

"I didn't even know I had grandchildren (the twins) until about four or five years ago," Richardson said.

Early last spring, Richardson got back in touch with his daughter and he communicated with Naomi Grangroth and his granddaughters weekly, either by phone or e-mail, he said. He was supposed to come to Temecula to visit them next month.

His voice choking with emotion, Richardson recalled the first time he saw Narissa and Nikita.

"I got to meet them at their funeral," he said. "Not a great way to meet your granddaughters."

Contact staff writer John Hall at (951) 676-4315, Ext. 2628, or jhall@californian.com. Comment at www.californian.com.