The jury that will decide whether to send Jessica Williams to prison for the next 120 years heard yet another expert testify that the 21-year-old was not impaired by drugs when she ran over and killed six teens. But they didn't hear from Williams herself.

The defense, which rested its case Tuesday afternoon, did not call Williams to the stand to testify about the accident. A former stripper with a genius-level IQ, Williams admits she smoked marijuana two hours before she fell asleep at the wheel and plowed into a group of juvenile offenders picking up trash alongside a Las Vegas highway.

Forensic toxicologist Michael Peat was the latest witness to say that Williams was not impaired by drugs at the time of the March 19 crash.

"She was not impaired by marijuana or THC at the time of the accident," testified Michael Peat, an Oxford-educated toxicologist. THC is the main ingredient in marijuana that causes mind-altering effects.

Peat said that he believed Williams was also not impaired by the drug Ecstasy, which Williams took more than eight hours before the afternoon accident.

The defense garnered support from Peat that the drugs did not contribute to Williams falling asleep behind the wheel.

"There is no evidence in the scientific literature that marijuana is a sedative," he said.

Under the influence of Ecstasy, a drive would tend to be "overaggressive," speeding or changing lanes excessively, according to Peat.

Williams was driving the 75 mph speed limit during her 40-minute ride from Valley of Fire state park to the accident scene. Witnesses testified that they saw her driving safely up until just moments before the accident.

Stepping down off the witness stand, Peat outlined the disparities in various blood tests on a chart perched on an easel before the jury box.

Williams' blood samples

Prosecutors, who had three samples of Williams' blood tested only weeks after the accident, say that the results show she was well over the legal limit for impairment for marijuana. Under Nevada law, a person is considered impaired if blood tests show more than 2 nanograms of marijuana.

The defense has claimed that 2 nanograms or higher would not signify impairment for a daily marijuana user like Williams since regular users have a baseline amount in their blood. To bolster this claim, defense lawyer John Watkins called nurses and emergency medical technicians who treated Williams to testify that she did not show any outward signs of impairment shortly after the crash.

But on Monday, Watkins challenged the accuracy of the blood test ordered by prosecutors. Calling to the stand toxicologist David Andrenyak, the defense presented a new set of drug test results to the jury that show that Williams had less than the required 2 nanograms of marijuana in her blood.

Prosecutors cried foul, charging that those test results were inaccurate since the tests performed by Andrenyak were completed 10 months after the crash, allowing the marijuana content in the blood to dissipate over time.

Watkins countered that according to studies done by Peat, marijuana levels in the blood are preserved over time as long as the samples are refrigerated.

The defense attorney grew incensed, charging that Williams was denied due process, after prosecutors revealed that they stored Williams' blood at room temperature before turning them over to the defense.

By calling Peat to the stand, Watkins attempted to assert that the tests ordered by prosecutors are unreliable. According to Peat, testing methods by the laboratory that conducted the prosecution's tests did not fit criteria of "accepted good lab practice today."

Peat also hinted that the blood tests ordered by the defense may still be reliable despite the room-temperature storage. Although the temperature affects the measure of marijuana, it does not affect the measure of marijuana metabolite, residual marijuana left over in the blood.

According to Peat, the difference of metabolite between the prosecution's tests and those of the defense was not significant.

On cross-examination, Peat was asked hypothetically about disastrous effects of drugs on a person's driving ability and was asked to define "disastrous."

"Any act that leads to the death of six people would be disastrous," he said.

To bolster its claim that this act, though "disastrous," was the result of a common mistake — nodding off at the wheel — the defense also called a sleep scientist to the stand. Dr. Mark Chambers testified that he thought the accident was caused by Williams falling asleep.

"Based on all the characteristics that we've examined — the nature of the crash, the timing of the crash, the position of the driver — I believe that all the characteristics point to the driver being asleep," Chambers said.

Chambers also testified that falling asleep while driving was fairly common in the Las Vegas area. He testified that a study reported "1,300 (sleep-related accidents) in a three-year period on the freeways surrounding Las Vegas."

Even though the long series of graphs, charts and scientific explanations presented by Chambers were somewhat of a sedative themselves, the defense scored points when Chambers testified that marijuana did not cause users to fall asleep.

"The studies have consistently shown that marijuana does not cause individuals to become sleepier," Chambers asserted.

In cross-examination, the prosecution tried to chip away at the expert's testimony, but didn't make much headway. Even a question concerning whether it was against "common sense" to get behind the wheel when sleep deprived ultimately backfired.

"If common sense is defined as 'what most people do' then most people do [drive when sleepy]," Chambers responded.

At another juncture, the courtroom erupted into laughter. When prosecutor Gary Booker asked him whether it was a good idea for Williams to drive while asleep, Chambers responded, "I don't believe that she was asleep as she got behind the wheel."

Testifying about his discussions with Williams about the circumstances of the accident, the sleep expert also offered a window into what the defendant says happened in the white van.

"(Williams) had already felt the vibrations from the car being on the shoulder of the roadway and that's what awakened her, rather than Tanya calling her name," recounted Chambers, referring to Williams' passenger, Tanya.

That second-hand account will be all the jury hears from Williams, who did not take the stand before Watkins rested the defense's case.

The prosecution recalled its own toxicology expert, Raymond Kelly, as a rebuttal witness. Kelly, who previously testified about the blood tests ordered by prosecutors, testified that the laboratory that performed the tests does not have adequate space to refrigerate blood samples.

He also said there are no industry-standards requiring that samples be refrigerated. Kelly reiterated that unrefrigerated blood dissipates over time, testifying that the findings of the defense-ordered blood tests was consistent with the deterioration rate.

Watkins called Peat back to the stand as a surrebuttal witness to reiterate that he found the prosecution's blood test findings unreliable. Peat also insisted that the blood evidence should have been refrigerated.

Before court ended on Tuesday, one juror sent a note to Gibbons expressing concern that prosecutors were making "faces and gestures" as if they were testifying. The juror said he thought that was "unfair to Jessica."

Out of the jury's presence, Watkins made a motion to dismiss the case, asking Gibbons at the very least to suppress the blood tests. While the defense lawyer argued that storing Williams' blood samples at room temperature was a due process violation, prosecutor Bruce Nelson suggested that Watkins deliberately delayed the test to give the marijuana time to dissipate.

"This is not a red herring," Nelson said. "It's a red whale."

Watkins retorted, "I'm not going to take that slander."

Gibbons, after ordering Watkins to sit down, denied the defense's motion but said it could renew the motion after the trial if Williams is found guilty.

Closing arguments are expected to begin Wednesday, and the jury is expected to begin deliberating late Wednesday.