The Baltimore Sun has obtained a copy of Freddie Gray’s autopsy report. Here’s what the medical examiner says happened to Gray in a Baltimore police van after his arrest in April. (Gillian Brockell/The Washington Post)

When six Baltimore police officers go to trial over the death of Freddie Gray, prosecutors will focus not on what the officers did, but on what they failed to do.

Unlike most criminal cases involving homicides — where someone is charged with shooting, hitting, stabbing or choking a victim — the one headed for trial in Baltimore in October involves an unusual and complicated concept: homicide “through acts of omission,” to quote the medical examiner in Maryland who performed Gray’s autopsy, according to a report obtained by the Baltimore Sun last week before it became public.

The medical examiner concluded that Gray, without a seat belt and handcuffed in the back of a police van, lurched when the van sped up or turned, striking his head and suffering a fatal neck injury. When Gray, 25, repeatedly asked for help, authorities said, he was ignored.

The autopsy report will be key to both sides in a closely watched case that is being scrutinized by legal experts and residents. Gray’s death in April sparked rallies that turned violent the day of his funeral, when riots, looting and fires forced authorities to declare a state of emergency and put Baltimore under curfew.

Clockwise from top left, Baltimore police officers William G. Porter, Garrett E. Miller, Caesar R. Goodson Jr., Edward M. Nero, Alicia D. White and Brian W. Rice. (Baltimore Police Department)

Legal experts said the challenge for prosecutors will be convincing a jury that each officer bears some responsibility in the death. The officer who drove the van is charged with depraved-heart murder, and three others face charges of involuntary manslaughter. Two others are accused of lesser crimes. Prosecutors said in court filings that investigators have amassed 300,000 digital files of evidence.

“Homicide by omission is a difficult legal theory in the criminal law,” said José F. Anderson, a professor at the University of Baltimore School of Law. “You have to show criminal responsibility by inaction.”

He added: “The real questions are going to be how can they pinpoint when the traumatic injury occurred or when [Gray] might have called out for help and for how long those cries were ignored.”

A spokesman for the Maryland Office of the Chief Medical Examiner declined to comment on the autopsy report or to provide a copy. The Baltimore City State’s Attorney’s Office did not return calls for comment. Attorneys for the officers either declined comment or did not respond.

Shackled at his feet, handcuffed behind his back and slid belly down into the van after his arrest, Gray may have been able to stand but unable to brace himself, the medical examiner concluded. He was thrown into a wall when the van suddenly sped up, slowed down or abruptly changed direction, according to the report. The medical examiner said that could have caused him to suffer a “high energy injury” — similar to an injury from a shallow water dive — that severely damaged his spinal cord.

“Due to the failure of following established safety procedures through acts of omission, the manner of death is best certified as homicide,” wrote Carol H. Allan, the assistant medical examiner, according to excerpts published in the Sun.

Several legal experts and pathologists said that based on what they read, they would have ruled Gray’s death accidental or undetermined because of the complexities and subjective nature of a case of homicide by omission.

“The concept of negligence or act of omission in classifying manner of death can be tough and is probably not done in a standard fashion around the country,” said Randy L. Hanzlick, professor of forensic pa­thol­ogy at Emory University in Atlanta.

Gray was arrested after he spotted officers and ran from a corner in West Baltimore. Prosecutors and officers disagree on the legality of the chase and over the legality of a spring-loaded knife found in Gray’s pocket after he was caught. Video shows officers dragging a seemingly limp Gray into a transport van.

The medical examiner wrote that she thinks Gray might have stood by bracing himself against the bench and was propelled head-first into a side wall.

“Mr. Gray was restrained with his wrists behind his back and at the ankles, was not belted with the safety belts that were present in the van, and due to an obstructed view of the roadway would have had trouble anticipating the van’s motion,” the report says. “Therefore, he was at risk for an unsupported fall during acceleration or deceleration of the van.”

Several legal experts interviewed said that it might be most challenging for prosecutors to prove the charge of second-degree murder, and they questioned the medical examiner’s report for being far from decisive. The medical examiner used phrases such as death “most likely caused” and “it is possible the neck injury occurred.”

“What they’ve identified is a single traumatic event that caused a death,” said Andrew C. White, a Baltimore defense lawyer and former federal prosecutor. “All the rest is just what prosecutors have written. The medical examiner takes it as the gospel truth, just assuming everything is true. No court hearing has tested the validity of any of the facts as alleged by the prosecutors.”

But Joseph E. diGenova, a former federal prosecutor in the District, called the autopsy report “unremarkable,” in that he said its clinical prose does not stray from accepted practice.

“A medical examiner is a detective,” he said. “That means they are trained to look with an independent eye not only at the toxicology, the chemistry, the physical injuries, but also at the setting, or the stage, of death.” He said that the medical examiner had to consider whether Gray had worn a seat belt, his positioning in the van, the way it was driven and whether any medical attention was rendered.

DiGenova said the defense would probably argue that “there are too many ways that this could have happened” to reach a firm conclusion.

Still, he said, the lack of restraints “is pretty damning evidence” of negligence. The union representing Baltimore police officers has said that regulations requiring prisoners to have on a seat belt were made mandatory just days before Gray’s arrest and that the rules were contained in a large packet of other directives that they said most officers never read. Prosecutors will have to prove that the officers were aware of the regulations and disregarded them, diGenova said.

William C. Brennan, a Maryland defense lawyer, said the biggest challenge for the state will be to prove the second-degree depraved-heart murder charge. According to jury instructions, prosecutors must convince a judge or jury that the driver of the van not only caused Gray’s death but also “created a very high degree of risk” to Gray’s life and, knowing that risk, “acted with extreme disregard of the life-endangering consequences.”

“It’s really stretching it and taking it too far based on these facts,” Brennan said of the murder charge.

Even the involuntary manslaughter charge will require prosecutors to prove officers were “grossly negligent.”

“This case is going to hinge enormously on a jury or judge’s estimation of what was in the individual mind of defendants at the time when they made these decisions,” said Adam Ruther, a Baltimore-based defense lawyer who worked as a prosecutor for Baltimore City before leaving in April.

Charges are filed against officers Caesar R. Goodson Jr., William G. Porter, Edward M. Nero and Garrett E. Miller, Sgt. Alicia D. White and Lt. Brian W. Rice.

Goodson faces the most serious charges­ — second-degree ­depraved-heart murder, involuntary manslaughter and assault, among other counts. Porter, Rice and White each face involuntary manslaughter and other charges. Nero and Miller face charges that include assault. All six also are charged with misconduct in office.

Keith L. Alexander contributed to this report.