Opening statements are set to begin Monday in the California trial for the man accused of killing college freshman Kristin Smart more than 25 years ago and his father, who allegedly helped his son hide her body. 

Smart was 19 when she disappeared from California Polytechnic State University's San Luis Obispo campus in 1996. Paul Flores was the last person seen with Smart on May 25, 1996, as he walked her home from an off-campus party where she allegedly got intoxicated.

To this day, the smiling face of Kristin Smart still looks out from a billboard. It once offered a $75,000 reward to help find the college student, but these days the billboard simply says: "Justice For Kristin."

Smart is still missing, while Flores is on trial more than a year after he was arrested on a murder charge along with his father, who is accused of helping hide her body.

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blonde kristin smart in overalls, bearded paul flores mugshot murder trial

Left: Still-missing college student Kristin Smart; Right: Paul Flores (FBI / San Luis Obispo County Sheriff's Office via AP)

Behind lattice work beneath the deck of Ruben Flores’ large house on a dead end street off Tally Ho Road – about 12 miles south of Cal Poly – archaeologists working for police in March 2021 found a soil disturbance about the size of a casket and the presence of human blood, prosecutors said.

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The blood was too degraded to extract a DNA sample. While a blood expert said it was human blood, the test used did not rule out the possibility it was from a ferret or ape, though court records said no remains of such an animal were found there.

White-haired Ruben Flores stares blankly in a mugshot

In this file photo provided by the San Luis Obispo County Sheriff's Office is Ruben Flores, 80, who was arrested in connection to the murder of college student Kristin Smart at his Arroyo Grande, Calif., home on Tuesday, April 13, 2021. (San Luis Obispo County Sheriff's Office via AP, File)

Opening statements are scheduled Monday in Monterey County Superior Court in Salinas in the trial of Flores and his father, Ruben Flores, who is charged as an accessory. Both men have pleaded not guilty.

The younger Flores, now 45, killed the 19-year-old during an attempted rape on May 25, 1996, in his dorm room at Cal Poly, where both were first-year students, prosecutors said. His father, now 81, allegedly helped bury the slain student behind his home in the nearby community of Arroyo Grande and later dug up the remains and moved them.

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Paul Flores with his mask half-removed in court Kristin Smart trial

Paul Flores puts on a new N95 mask Aug. 3, 2021, at a preliminary hearing in San Luis Obispo, Calif. Kristin Smart disappeared during her freshman year at California Polytechnic State University in San Luis Obispo more than 25 years ago. (David Middlecamp/The Tribune (of San Luis Obispo) via AP, Pool,File)

Paul Flores had long been considered a suspect in the killing, but prosecutors only arrested him and his father in 2021 after the investigation was revived.

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San Luis Obispo Sheriff Ian Parkinson acknowledged detectives' missteps over the years, and he credited a popular podcast about Smart's disappearance called "Your Own Backyard" for helping unearth new information and inspiring witnesses to speak with investigators.

Ruben Flores in a suit and Paul Flores in a jumpsuit appear over Zoom in a court proceeding

In this April 15, 2021, file photo defendants Paul Flores, top left, and his father, Ruben Flores, bottom right, appear via video conference during their arraignment in San Luis Obispo Superior Court in San Luis Obispo, Calif.  (AP Photo/Nic Coury, File)

Smart’s remains have never been found and the mystery of how she vanished from the scenic campus tucked against a verdant coastal mountain range is likely to be central to the trial. 

A San Luis Obispo Superior Court judge ordered the pair to trial after a 22-day preliminary hearing in which he found a "strong suspicion" the father and son committed the crimes with which they were charged, that a grave existed under Ruben Flores' deck and that it once held Smart's remains.

Ruben Flores with white hair wearing a mask appears in court Kristin Smart trial

Ruben Flores, father of Paul Flores, sits in court Monday, Aug. 2, 2021, in San Luis Obispo, Calif.  (David Middlecamp/The Tribune (of San Luis Obispo) via AP, Pool,File)

Prosecutors, defense lawyers and San Luis Obispo sheriff’s deputies are constrained by a court order prohibiting them from discussing the case.

Attorney Harold Mesick, who represents Ruben Flores, previously said the evidence unearthed was ambiguous. He said that soil under the deck had been dumped there after being excavated to lay a foundation nearby.

A billboard displaying missing person Kristin Smart near a tree and houses

Sept. 7, 2016: A sign outside the James R. Murphy, Jr. law office in Arroyo Grande, Calif., offers a reward for information in the 1996 disappearance of Kristin Smart. (AP)

The younger Flores downplayed his interactions with Smart when he first spoke with police three days later, saying she walked to her dorm under her own power, though other witnesses said that she had passed out earlier in the night and Flores helped hold her up as they walked back to campus.

Flores had a black eye when investigators interviewed him. He told them he got it playing basketball with friends, who denied his account, according to court records. He later changed his story to say he bumped his head while working on his car.

Kristin Smart smiling in a blue swimsuit by the beach

Kristin Smart went missing on May 25, 1996, while attending California Polytechnic State University, San Luis Obispo and has not been heard from since. ( Axel Koester/Sygma via Getty Images)

At a preliminary hearing last year, prosecutors presented evidence that four cadaver dogs stopped at Flores’ room and alerted to the scent of death near his bed.

The judge ruled in favor of a defense request to move the trial out of San Luis Obispo County because it was unlikely the Flores' could receive a fair trial with so much notoriety in the city of about 47,000 people. Instead, the case was moved 110 miles north to Salinas.

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Separate juries were selected to weigh the evidence against each defendant. The trial is expected to last about four months.

The Associated Press contributed to this report.