Investigators in California said they did not locate the remains of Kristin Smart — a 19-year-old who went missing in 1996 — at a property where earlier soil testing suggested the presence of human remains.

“We did not recover Kristin Smart,” the San Luis Obispo County Sheriff’s Office said in a statement on Saturday afternoon. “Detectives will be evaluating any evidence we have recovered to aid in the investigation.”

Ms. Smart was a student at California Polytechnic State University, San Luis Obispo, who vanished after an off-campus party during Memorial Day weekend in 1996. The case has spanned nearly 30 years.

It took more than two decades for Paul Flores, who was also a student at the time, to be arrested in 2021 and ultimately convicted of her murder the following year.

Even after successfully prosecuting her murder, officials vowed to find Ms. Smart’s remains.

“It wasn’t a finale for us,” Ian Parkinson, the sheriff of San Luis Obispo County, said at a news conference on Friday about Mr. Flores’s conviction. “The case was not over. The reality was that Kristin is still missing.”

On Wednesday, the sheriff’s office began searching a property owned by Mr. Flores’s mother in Arroyo Grande, Calif., just south of San Luis Obispo.

The property had been searched multiple times before, Mr. Parkinson said, but a new lead — and advancements in ground penetrating radar and soil testing technology — led them back to the home.

Early testing of the soil outside the home proved promising. The results indicated that human remains had once been there — or could still be there, Mr. Parkinson said.

Investigators continued searching the property into the weekend, and Mr. Parkinson said they would not stop until they found Ms. Smart’s remains or conclude that she was not there.

The case has long haunted California. Mr. Parkinson led the renewed efforts when he was elected sheriff in 2011, promising the Smart family to aggressively pursue the case.

Mr. Flores told investigators at the time of Ms. Smart’s disappearance that he accompanied her back to campus after the party on May 25, 1996. Mr. Flores, who was also a student, was a person of interest early in the investigation. He was not criminally charged at the time.

Extensive searches of the San Luis Obispo campus and surrounding areas involved helicopters, horseback search parties and excavators but yielded little. Billboards and missing posters offering rewards went up all around the area.

The Smart family sued Mr. Flores in 1997 in a wrongful-death lawsuit, but he refused to testify in a deposition, citing the Fifth Amendment.

The investigation into Mr. Flores, now 49, gained momentum in 2020, when four locations tied to him were searched by the police, including his home in Los Angeles. It culminated with his arrest and murder charge in April 2021. Prosecutors said he killed Ms. Smart during an attempted rape.

His father, Ruben Flores, was also arrested and accused of helping his son hide Ms. Smart’s body. The elder Flores was ultimately acquitted, but his son was convicted of the murder in Oct. 2022.

In 2023, Paul Flores was sentenced to 25 years to life. He has maintained his innocence and has appealed his conviction.

The Flores family has not been cooperative in the search for Ms. Smart, Mr. Parkinson said.

At the Friday news conference, he indicated that the search would continue even if they did not recover Ms. Smart’s remains at the property in Arroyo Grande.

“If it doesn’t net the results we want, we have done a great job of changing the direction and moving onto something else and pursuing that till the end,” Mr. Parkinson said. “And that’s what we’ll continue to do.”