SOUTH LAKE TAHOE, Calif. — It is unlikely, but not impossible, that witnesses who say they saw Alexander Olive alive after January 1986 were able to accurately identify the boy based solely on a photograph, according to an expert's testimony in the Ulysses Roberson murder trial this week.

El Dorado County prosecutors have accused Roberson of first degree murder in the disappearance of Olive — Roberson's 4-year-old son — from a Tahoe Keys home in late December 1985 or early January 1986. No body has ever been found.

Scott Fraser, a University of Southern California professor and an expert in eyewitness testimony, told jurors on Tuesday that a person needs to repeatedly view someone's face at different angles and under different lighting conditions to be able to reliably identify them.

Prosecutor Patricia Kelliher called Fraser to the stand to rebut testimony from nine defense witnesses who said they believe they saw Olive alive years after Roberson allegedly killed him.

Most of the witnesses based their reported sightings of Olive on an episode of the television show “Unsolved Mysteries” featuring a segment on Olive's disappearance. The segment included several real and one age-progressed photo of Olive.

Although Roberson's attorney Monica Lynch, had not yet finished her case at the time of Fraser's testimony, El Dorado County Superior Court Judge Suzanne Kingsbury allowed Kelliher to call Fraser out of order as a matter of judicial economy.

In addition to needing multiple viewings of someone's face to accurately identify them, Fraser detailed several factors that decrease a person's ability to identify a person based on a photograph.

Even when people are asked to immediately identify a person in a photo after viewing them for 10 minutes, they are only able to accurately identify that person 66 to 75 percent of the time, Fraser said. When asked to identify a person five to six hours after viewing them for 10 minutes, people's identification accuracy drops to 25 to 33 percent, Fraser said.

While people are very good at identifying the “distinctive cues” of a person — what sets them apart from other people — they are not very accurate at identify details of a face. Distinctive cues include features like unusual haircuts, scars, tattoos, freckles and light eye color, Fraser said.

Several of the witnesses who said they saw Olive after January 1986 said that they remembered seeing the boy because it was unusual to see a black person with red hair.

A mistake in at least one of the airings of the “Unsolved Mysteries” show — where a picture of one of Olive's older brothers is shown while the narrator discusses Olive's disappearance — also could have hindered accurate eyewitness identification of Olive, Fraser said.

“Any time you have multiple targets, the ability to identify a single one decreases,” Fraser said.

Following Fraser's testimony, Kingsbury denied a motion for mistrial made by Lynch.

Lynch contended Fraser implied the defense witness sightings were false, even though Fraser's testimony was supposed to be limited to generalities about eyewitness testimony. She pointed to testimony where Fraser said more than 99 percent of witnesses of who view photo lineups identify a person, even when the person they're being asked to identify is not in the lineup.

“I think he went too far,” Lynch said.

The trial resumes on Wednesday.