UPDATE: Prosecutors rested their case Tuesday in the trial against Benjamin Brewer. 

The jury heard testimony from 15 state witnesses over the last two days. 

Tuesday's testimony centered around scientific evidence and expert witness testimony. 

Collegedale Police Chief Brian Hickman showed the jury how he instructs people to perform physical tests when trying to determine if someone is under the influence of an illegal substance. 

"You take a series of small steps around, like so, so you can go back to your next nine [steps]," Hickman testified.

He said Brewer showed clues in every test of being under the influence hours after his truck crashed into stopped traffic, killing six people. 

But defense attorney's argued the science behind the testing is unreliable, trying to prove Brewer was not high on meth. 

"If someone has a small amount of substance in their blood, does that necessarily mean they're impaired?" Defense attorney Jay Underwood asked. 

"No," Hickman replied. 

Testimony shifted focus to the truck Brewer was driving. 

Lt. John Harmon with Tennessee Highway Patrol said he didn't find an issue with the brakes in his post-crash inspection, despite Brewer's statements to police. 

"We took air and added it to the brake chamber on the right side and they both worked properly," Harmon testified. 

Evidence on the scene backed up Harmon's testimony. 

Chattanooga Police Officer Joe Warren said tire marks left on the roadway showed Brewer did not apply his brakes. 

"All I could see were impact marks, based on the types of tire marks I was seeing. Lot of impact, in the impact area, the first initial impact area," Warren testified. 

The jury also heard testimony from TBI Forensic Scientist Miranda Quinn, who tested Brewer's blood at the state crime lab. 

She says Brewer's blood tested positive for meth and amphetamine. 

Hamilton County Assistant Chief Medical Examiner Steven Cogswell performed autopsies on the six victims and ruled all the deaths as accidental. 

All evidence for the jury to weigh when deciding Brewer's fate. 

Wednesday will be the defense's turn to call witnesses. 

With the morning session concluded, testimony continues this afternoon in the trial of Kentucky truck driver Benjamin Brewer, accused of causing a crash that killed six people in 2015. 

Much of the testimony Tuesday has centered around the drug tests performed hours after the crash. 

Collegedale Police Chief Brian Hickman testified that Brewer failed several tests and that his pulse and blood pressure were high, leading to the conclusion that he was high on a stimulant. 

But the defense argues the science behind the testing isn’t reliable, calling it quote “junk science” because it’s only reliable 33% of the time. 

This testimony is crucial in this case because the case centers around whether or not Brewer was high on meth at the time of the crash. 

The defense says brewer fell asleep behind the wheel. 

Brewer is charged with 6 counts of vehicular homicide from the crash; there are 6 other charges related to injuries from the crash. If convicted, Brewer faces up to 72 years in prison. 

Channel 3's Michelle Heron is in the courtroom and tweeting live updates from the trial. Her posts can be seen below.