By Andrew Chung

The U.S. Supreme Court on Monday agreed to hear a California woman's bid to overturn her conviction for smuggling drugs across the U.S.-Mexican border in a dispute over the scope of prosecution expert witness testimony presented to undermine her defense that she acted unwittingly as a "blind" drug mule.

The justices took up Delilah Guadalupe Diaz's appeal that she filed after a lower court rejected her bid to exclude the expert witness's testimony that had cast doubt on her claim that she did not know that methamphetamine valued at $368,550 was hidden in the door panels of the car she was driving.

A jury in federal court in San Diego found Diaz guilty in 2021 of illegally importing the methamphetamine, a crime that required knowledge that she knew the drugs were in the car.

People who smuggle drugs across borders, sometimes called "mules," may do so for profit but also sometimes do it unwittingly, transporting illegal substances that have been planted on them. These individuals are often referred to as "blind" mules.

U.S. District Judge Anthony Battaglia allowed the prosecution's expert witness, a Homeland Security special agent, to testify that "in most circumstances, the driver knows they are hired." The expert also told the jury that drug-trafficking organizations generally do not entrust large quantities of drugs to unknowing couriers.

Diaz's attorneys contend that the testimony violated the longstanding Federal Rules of Evidence governing the types of evidence allowable in legal cases. They have argued that the testimony broke a rule barring expert witnesses from offering opinions on the "mental state" of defendants related to an alleged offense - whether they knew they were committing a crime.

Border inspectors ordered Diaz, a resident of Moreno Valley, California, to roll down a window of the Ford Focus vehicle she was driving and heard a "crunch-like" sound, later finding 56 packages containing 24.5 kilograms of pure methamphetamine. Diaz denied knowledge of the drugs.

She carried two cellphones - one locked that she could not open - and claimed that the car belonged to a boyfriend she had visited in Mexico whose phone number and residence she could not identify. The car also had a hidden GPS device. Diaz was sentenced to seven years in prison.

The San Francisco-based 9th U.S. Circuit Court of Appeals in January rejected her appeal.

"We have allowed such testimony so long as the expert does not provide an 'explicit opinion' on the defendant's state of mind, and the expert did not do so here," the 9th Circuit stated.

Diaz's lawyers urged the Supreme Court to take up the appeal because allowing generalizations during a trial that suggest a defendant must be guilty is "grossly unfair."

The court will hear arguments during its current term and is expected to decide the case by the end of June.

(Reporting by Andrew Chung; Editing by Will Dunham)