Mostrong, 21, was driving his van with a group of passengers in the western Imperial Valley desert when the van struck and killed Michelle.
Contreras on Monday said he has seen a number of Mostrong's friends in the courtroom during the trial.
Contreras said if Reed's statements are true, he "will take whatever steps are necessary" to deal with any intimidation Reed or any other witness may face.
Espinosa then shot back at Contreras that he was throwing blame at Mostrong without facts. He questioned whether Contreras was doing so because he knows the Reed family.
Contreras responded that his warning was meant to assure justice.
Espinosa questioned whether a police report had been taken on the "keying" incident.
Deputy District Attorney Wayne Robinson said such a report had been taken. He added another witness has faced intimidation.
Contreras said the issue with that witness would be discussed when the witness takes the stand.
The controversy over Reed's allegations came after he offered testimony that he was a passenger in the van the night it struck Michelle.
Reed's testimony contrasted sharply with that given earlier Monday from another witness in the van and with testimony given last week from a third witness in the van.
Reed testified he had seen Mostrong drinking beer from a "beer bong" before Mostrong got into the van for the drive that would end in a fatal accident. Reed said he was the one who brought the "beer bong" to the party.
Another witness, Ismael Castillo, 21, of El Centro, said he had not seen Mostrong drinking, although he added he did not see much of Mostrong before the van ride.
On Wednesday, Jason Jimenez an El Centro resident who was 20 at the time of the Oct. 7 accident, testified he had not seen Mostrong drinking that night.
Reed testified he recalled there were six people in the van while Castillo and Jimenez testified they could only recall five.
Reed testified there was alcohol in the van, including beer and a mixed drink called "jungle juice." He said he knew there was "jungle juice" because it spilled on him after the crash.
Castillo testified earlier Monday he had not seen any alcohol in the van. Jimenez said the same thing when he testified Wednesday.
Reed further testified that while it seemed Mostrong was in control of the van, he was driving "wild."
"I wouldn't have driven like that," he said, adding, "I was scared."
Castillo said from where he was sitting in the back of the van it seemed Mostrong was driving safely for the road and the time of night. He testified he would have driven in the same manner.
Reed said he told Mostrong to slow down and testified he saw lights ahead as they neared the golf cart that Michelle and two others were near.
Castillo and Jimenez both testified they did not remember seeing lights on Wheeler before the crash.
The prosecution has contended the golf cart had its lights on at the time of the accident and that Mostrong did not attempt to swerve or slow down to miss the golf cart.
The defense has argued the golf cart was stopped on the wrong side of the road in the path of oncoming vehicles and the cart did not have its lights on, which would have made it impossible for Mostrong or anyone else to see it.
Reed further testified on Monday that after the crash, the van stopped and everyone got out. He claimed Mostrong knew he had hit at least one person and made a statement to that effect outside the van.
Reed said he told Mostrong they had to go back to the crash site, but, he claimed Mostrong did not listen.
Reed said he ran back to the "keg" party where they had been, got in his truck and drove away. He said as he ran back to the party, he heard the van drive away, although he did not know who was driving it.
Reed further testified he did not contact the California Highway Patrol until the next day, after he had taken a shower, gone to church and told his parents about the collision.
Castillo and Jimenez testified they never contacted the Highway Patrol but the Highway Patrol contacted them later in the month.
Espinosa pointed out Reed said things before the jury that he had not told the Highway Patrol or the defense when interviewed prior to the start of the trial.