What led to mistrial for Officer William G. Porter in Freddie Gray case

Prosecutors and defense lawyers have sparred for days on many issues in the trial of Officer William G. Porter, who faced four charges in the April death of Freddie Gray. Jurors have not specified why they were deadlocked, but here's a summary of key conflicts that confronted jurors:

When Gray's injury occurred: Dr. Carol Allan, the assistant state medical examiner who performed the autopsy on Gray, said he was injured between the second and fourth stops. She would not have ruled the death a homicide, she said, had Porter or the van's driver, Officer Caesar R. Goodson Jr., sought medical care for Gray at the fourth stop.

A defense witness, Dr. Vincent DiMaio, said Gray's catastrophic injury must have occurred between the fifth and final stops of the van. Had it occurred any earlier, Gray would not have been able to ask for help, kneel, or move his body in any way.

Seat-belting: Prosecutors argued that Baltimore Police Department rules clearly require officers to seat-belt arrestees. Porter's lawyers argued that police rarely seat-belted prisoners and that the departmental rule — updated just weeks before Gray died — was not well-known.

Medical care: Prosecutors argued that departmental rules require officers to provide medical care to those in custody when they ask for it. A police investigator said Porter told her that Gray complained he could not breathe; the comment came in an unrecorded, informal phone conversation days after Gray was hurt

Porter testified in his defense that he didn't think Gray was badly hurt, and the defense contended that the investigator misunderstood Porter's comment in the phone conversation.

Officer discretion: Jurors heard conflicting testimony about the responsibilities of police officers, a key issue for the panel when considering whether Porter's actions were legally "reasonable." Prosecutors said officers are trained and duty-bound to act, while the defense countered that situations aren't always clear-cut and officers are allowed wide discretion.

Vagueness of charge: Michael Serota, an attorney who works on criminal code reform, studied cases in which defendants were accused of involuntary manslaughter and depraved-heart murder, the charge faced by Goodson. Serota was surprised by the lack of clarity in court rulings in Maryland.

Judges have called the state's laws "treacherously ambiguous," and "perplexing," Serota wrote in a recent article. In Maryland, he noted, the criminal code doesn't define manslaughter other than to say it's a felony and to outline the penalties.

Serota said unclear laws are problematic for a number of reasons, including that they make the court's work more difficult and complicate the jury's job of determining what's illegal.