A drug expert testified about the influence marijuana and Ecstasy may have had on Jessica Williams when she fatally plowed into six teens while driving her minivan.

As prosecutors wound down their case against Williams on Wednesday in a Las Vegas courtroom, toxicologist Raymond Kelly testified that marijuana slows reaction times — and could even make someone who is under its influence fall asleep.

Prosecutors say Williams, 21, was impaired by drugs on March 19, 2000, when she drove into the group of teens picking up trash alongside a Las Vegas highway. While the defense doesn't deny that Williams used drugs, it contends she was not impaired but merely fell asleep behind the wheel. Prosecutors, however, argue that asleep or awake, Williams was impaired and should be convicted of driving under the influence. If found guilty, the former stripper with a genius-level IQ could face 120 years behind bars.

Williams

During direct examination by prosecutor Gary Booker, Kelly explained the combined effects of marijuana and Ecstasy, a mild hallucinogen.

Kelly said that those under the influence of any hallucinogen are often alert and talkative — and that they have a tendency to be more aggressive behind the wheel.

"It causes your senses to not give an accurate perception of what's happening in the real world," Kelly testified.

Prosecutors could claim that Williams exhibited this type of behavior at the scene, based on eyewitness testimony that, shortly after the crash, she kept asking if she was dreaming.

Williams' blood tested well beyond the levels Nevada law considers necessary for impairment. Defense lawyer John Watkins, who tried to argue in pretrial proceedings that the law itself is unconstitutional, got Kelly to admit on the stand that research in the field shows there is no evidence to support a correlation between levels of THC, the mind-altering substance in marijuana, and impairment.

After reluctantly conceding that point, Kelly shot back, "But they don't say in their study that marijuana doesn't impair driving, do they?"

Kelly had previously said that, after 10 to 15 minutes, the THC level hits a "peak" — or its maximum amount.

Blood tests revealed that Williams had 5.5 nanograms of marijuana in her blood more than an hour after the crash — almost three times the 2.0 nanograms needed to legally prove impairment. Watkins contends that, as a regular marijuana user, Williams had a higher baseline blood level — 5.5 nanograms to a regular user would not be as strong as to an infrequent user.

But Kelly rebuffed that notion, saying he's never seen a case with a baseline as high as 5.5 nanograms.

He also added that blood tests could underestimate the effect of marijuana, since the THC remains in the brain after it leaves the bloodstream.

Watkins contends that, of the hundreds of chemicals in marijuana, only some are mind-altering, making the blood test an inaccurate measuring tool of how high a person is.

During the heated cross-examination, Kelly yielded slightly when Watkins suggested someone under the influence of marijuana could conceivably operate a car safely — but only in certain circumstances, Kelly said. A person could parallel park safely, he said, but driving 70 miles per hour on a highway, as Williams was, is another story.

Kelly also admitted under cross-examination to smoking marijuana himself, adding, "Hopefully the statute of limitation has run out by now."

The witness stand wasn't the only source of tension for the defense lawyer. Watkins, who was threatened with jail time by Judge Mark Gibbons during opening statements, was admonished repeatedly during his questioning of Kelly.

Gibbons once asked Watkins to stop being sarcastic to Kelly, and another time told both Watkins and Booker that he was "sick of this nonsense." Booker later elicited a laugh from courtroom observers by saying that he'd be "objecting to everything" when asked by Gibbons if he wished to voice an objection on one issue.

The jury also heard testimony previously given by Tania Ozarek-Smith, the passenger in Williams' van at the time of the crash. Her grand jury testimony was read in the courtroom after both sides were unable to locate her to testify.

"I looked over at Jessica and she was sleeping," she testified. "She woke up, and then it was too late."

Testimony is expected to last through the end of the week.