Under questioning from Robinson, Ota said Mostrong's blood tested positive for THC, the active element of marijuana.
Ota further said based on the test it was his opinion the marijuana was used three to four hours before Mostrong's blood was drawn.
Ota said he could not tell whether Mostrong had used marijuana before he drove his van down Wheeler Road for the drive that would end in the fatal crash.
The collision occurred just before midnight Oct. 7. Mostrong turned himself into authorities on Oct. 8, about 17 hours after the accident.
The prosecution is asking the jury to find Mostrong guilty of vehicular manslaughter, hit-and-run and driving under the influence.
Two witnesses called by the prosecution to testify about Mostrong's blood levels could not say for certain whether he was under the influence at the time of the accident, due to the length of time between the accident and the blood sampling.
On Wednesday, Javed Khan, a Department of Justice analyst who studied Mostrong's blood for alcohol content, said he did not know whether Mostrong was under the influence of alcohol when the accident occurred.
He said the blood sample drawn had an alcohol level of .00. However, he said if Mostrong last drank alcohol 17 hours before his blood was taken, he would not have alcohol in his blood when it was drawn.
On Friday, Ota said Mostrong clearly had marijuana in his system and he was certain the marijuana had been used only three to four hours before the sample was taken.
That means at some point after the collision Oct. 8, Mostrong used marijuana. It does not answer the question of whether he used the drug prior to the accident.
Before the collision Mostrong had been at a "keg" party in the Superstition Mountains where, witnesses have testified, people were using marijuana and drinking beer. Two witnesses have testified they saw Mostrong drink beer and use marijuana at the party.
Other witnesses have said they could not recall whether Mostrong drank beer or smoked marijuana.
California Highway Patrol Officer Pablo Torrez, lead investigator in the case, testified that some of the same witnesses who testified they could not recall whether Mostrong was drinking or smoking marijuana had said in interviews with him they did see Mostrong drink beer beer and that he was passed a joint.
Those witnesses testified Torrez intimidated them and put words in their mouths.
The chain of events that has been laid out in the case is that Mostrong got in his van with others from the party and drove down Wheeler Road.
Michelle was in the area with a group of friends.
She and her friend Charlotte Lang had driven in a golf cart along Wheeler and the cart had come to a stop on the wrong side of the road.
Another friend, Eric Welter, who had been riding a motorcycle, was with them after the cart stopped
Shortly before midnight, Mostrong's van collided with the vehicles on the road, including the motorcycle and the golf cart. Michelle and Eric also were struck. Michelle died at the scene; Eric survived his injuries.
The van reportedly came to stop away from the accident site. Those who were in the van got out, and most returned to the party without checking on what the van had hit, witnesses have said.
Mostrong and another person who had been in the van, Kristoffer Purdy, drove away from the scene. Purdy testified he drove the van from the scene with Mostrong in the passenger seat.
The two drove until the van suffered a punctured tire. They then walked several miles from the desert into Imperial.
About 4 the next afternoon, Mostrong, accompanied by an attorney, turned himself into the Highway Patrol.
The defense has contended Mostrong was not under the influence of alcohol or marijuana and there is no proof he drank or used the drug at the party.
The defense has contended there was no way Mostrong could have seen the golf cart in the road because the cart did not have its lights on.
The prosecution has argued the lights on the cart were on.
When Espinosa questioned Ota on Friday about the marijuana in Mostrong's system, he focused on the fact the test did not show whether Mostrong was impaired when he drove his van.
Ota said his toxicology test cannot determine if a person is under the influence of a drug; it simply shows whether a person has a drug in his or her system.
"I would not render an opinion on that," Ota said of the question of whether Mostrong was an impaired driver.
The trial will continue Monday at 9:30 a.m. with Espinosa calling his first witness.
Staff Writer Darren Simon can be reached at 337-4082.