Why Kelsey Berreth Murder Trial May Not Be Slam-Dunk Case Against Patrick Frazee

(CBS NEWS) - Jury selection is set to begin Monday in the murder trial of the man accused of killing Colorado mother Kelsey Berreth. The 29-year-old woman was last seen in a grocery store with her baby on Thanksgiving Day last year. Her body was never found. Berreth's fiancé, Patrick Frazee, is charged with her murder.

At the center of this trial is Krystal Lee, a nurse who was allegedly having an affair with Frazee. She claims she cleaned up the murder crime scene because she wanted to please Frazee – and because she feared him.

As prosecutors prepare their star witness for the trial against Frazee, legal experts say it may not be a slam-dunk case.

"This case really does live or die on Krystal Lee," University of Colorado law professor Aya Gruber told correspondent Nikki Battiste. "There's no body and there's no murder weapon."

Lee, an Idaho nurse, was allegedly Frazee's secret girlfriend. She told authorities Frazee tried to get her to kill Berreth three different times – once with a poisoned Starbucks drink.

Lee said, in the end, it was Frazee who murdered the mother of his child at Berreth's home on Thanksgiving Day by beating her with a baseball bat. But male DNA found at the crime scene does not match Frazee.

Battiste asked Gruber, "How would you describe the physical evidence in this case so far?"

"I would describe the physical evidence that I know of to be pretty weak," Gruber replied.

At trial, Frazee's defense may point the finger at Lee, suggesting she's fabricated parts of her story before. "They will absolutely paint Krystal Lee as a liar," Gruber said, "and in fact there is evidence that initially she did lie to investigators. She said that she had no idea who Patrick Frazee or Kelsey Berreth were."

After Berreth's disappearance, investigators say her cellphone pinged 800 miles from her home in Woodland Park, Colorado, near Gooding, Idaho. That's where investigators found Lee, who eventually admitted to destroying Berreth's phone, and also getting rid of Berreth's keys and a gun, all to help Frazee cover his tracks.

In an interview for "48 Hours," Laura Stutzman, who lives in Lee's Idaho community and has known her since she was a teenager, asked, "Why? Krystal had it all. She had it all. There's no sentence imposed by a judge that can actually be more harsh than what her community will put on Krystal."

In exchange for her testimony, Lee agreed to plead guilty to evidence tampering. She faces a maximum of three years in prison, but if her story falls apart, her deal is off the table.

Frazee has entered a not guilty plea, and faces life in prison if convicted.