Officer Caesar R. Goodson, Jr., 47, drove the transport van carrying Freddie Gray. "Despite stopping for the purpose of checking on Mr. Gray's condition, at no point did he seek nor did he render any medical assistance for Mr. Gray," said Baltimore State's Attorney Marilyn Mosby on May 1, 2015.  Goodson, with the Baltimore Police Department since 1999, was charged with second-degree depraved-heart murder, involuntary manslaughter, second-degree assault, two counts of manslaughter by vehicle and misconduct in office.
CNN  — 

The assistant medical examiner who called Freddie Gray’s death a homicide testified Friday in the trial of the police officer who was driving the van in which the doctor determined Gray suffered his injuries.

Dr. Carol Allan stated “with a reasonable degree of medical certainty” that the fatal spinal cord injury that Gray suffered was distinctive, that it could only happen in a certain way and that it occurred while he was in the van.

Her testimony is key in the prosecution’s case against Baltimore Police Officer Caesar Goodson, who faces the most serious charges in a controversial case that sparked days of unrest in Baltimore and protests across the country after Gray was arrested and died in April last year.

Allan said she based her opinion on her autopsy of Gray, interviews with witnesses and an examination of the van where she said Gray was placed face-down on the floor with feet shackled and his hand tied behind his back.

Goodson’s lawyer, Amy Askew, prodded Allan with questions designed to suggest Allan had stretched her medical opinion to classify Gray’s death as a homicide.

“Mr. Gray’s death was while he was in custody,” said Allan, who testified she has performed 5,100 autopsies since being employed by the Maryland medical examiner’s office in 2003. “This is not an accident.”

Prosecutors contend Goodson took the unbelted Gray on a “rough ride” – which was described as punitive measure police used against unruly subjects.

They contend Goodson drove so radically that he blew through a stop sign and veered into another lane of traffic because of the speed he was traveling.

Allan testified about a picture showing the injuries to Gray’s spinal cord, pointed out where the spinal cord was pinched and reiterated that the cause of death was a neck injury consistent with a homicide.

Goodson is the third of six officers tried in the case.

On Monday, the officer elected for a bench trial, meaning his fate will be decided by the same judge who acquitted a Baltimore officer last month on charges related to Gray’s controversial death last year.

 Baltimore Police Officer Edward Nero (C) arrives at the Mitchell Courthouse-West on the day a judge issued an acquittal in his case.

Judge Barry Williams on May 23 acquitted Officer Edward Nero, one of three bicycle officers involved in the initial police encounter with Gray.

Goodson faces charges alleging second-degree depraved-heart murder, second-degree assault, misconduct in office, involuntary manslaughter, manslaughter by vehicle (gross negligence), manslaughter by vehicle (criminal negligence) and reckless endangerment.

Defense lawyers contend that Gray’s injuries were caused in part by his own agitation and thrashing around in the van. They contend that the medical examiner initially called the death a “freakish accident.”

Prosecutors have said that Gray complained of having trouble breathing and asked for medical help as he was driven in a police van. When he arrived at a police substation, he was unconscious. A week later, Gray died at a hospital from a spinal injury.

Riots erupted in Baltimore last year after Gray’s funeral.

Baltimore Officers Charged Freddie Gray

In December, a mistrial was declared in the case of William Porter, the first officer to go on trial in the in connection with Gray’s arrest and death. He is scheduled to be retried in September.

Three other officers have yet to stand trial – Officer Garrett Miller, Lt. Brian Rice and Sgt. Alicia White.

Rice’s trial will start July 5, Miller’s on July 27 and White’s on October 13.

Goodson’s trial continues Monday.

CNN’s Ray Sanchez contributed to this report