Updated for testimony from Dec. 9.

Officer William Porter is the first of six Baltimore police officers who stand accused of playing a role in the death of Freddie Gray, a 25-year-old black man who died after injuries sustained in the back of a police van while he was handcuffed and shackled. Porter, who joined the police force in 2012, faces charges of involuntary manslaughter, second-degree assault, and misconduct in office. Essentially, prosecutors want him held accountable for failing to put Gray in a seatbelt, and failing to call medical aid. Here's a breakdown of some key questions that have come up in court.

Why didn't Porter seatbelt Gray?

It's the policy of the Baltimore Police Department to seatbelt those in custody, but in his opening statement defense attorney Gary Proctor suggested few do. He told jurors Porter was "just like every other officer." Officers placed Freddie Gray face down in the police van, handcuffed and with his legs shackled. Porter checked on Gray at a later stop, and helped pull him up on to a bench, but still did not seatbelt Gray. On April 9, 2015, three days before Gray's arrest, new orders were emailed to officers that required seatbelting detainees without exception. But defense lawyers say it was "buried" in an 80-page attachment. The police department's head of IT says that Porter received that email, but he can't tell whether he read it.

This issue isn't new. There are a number of previous instances in Baltimore of people dying or becoming paralyzed after riding in Baltimore Police vans without being seatbelted.

Update, Dec. 9:

When he took the stand on Wednesday, Porter said he never seatbelted detainees in police vans. Porter said that the back of the van is a "pretty tight" space in which to maneuver, and that seatbelting detainees could involve his gun getting close to the hands of the prisoner — though Gray's hands were cuffed behind his back.

Why didn't Porter call a medic?

One of the most crucial parts of this case took place during the fourth stop of the police van, when Officer Porter checked on Gray. Porter has told police investigators that Gray asked for help, said he couldn't breathe and couldn't move. Porter said he helped lift Gray onto the bench in the van. He says he asked if Gray needed a medic, and Gray said yes. When police investigators asked why he didn't call one, Porter said, "I thought he was faking," in part because Gray couldn't explain what was wrong. The state's medical examiner and an expert neurosurgeon testified that by that point, Gray's spinal injury would mean he was struggling for air, finding it difficult to talk. Porter also told investigators that medics "don't want to take prisoners if we already have transport" and that police could take a detainee to the hospital themselves. But prosecutors say it would have been as easy as pressing a button on his chest for Porter to call for a medic.

Update, Dec. 9

During his testimony, Porter said he didn't call a medic because Gray didn't seem hurt. According to Porter, Gray spoke in a normal tone of voice and made eye contact at the van's fourth stop, and though Gray said "yes" when Porter asked if he needed a medic, Porter said Gray was unable to give him any reason for a medical emergency.

Porter further testified that medics can take a long time to arrive, so it made more sense for the van to take Porter directly to the hospital — which Porter says he suggested to Caesar Goodson, the driver. The trip to the hospital was needed to clear Gray for central booking, which wouldn't accept Gray if he told them he needed medical help.

Who was responsible for Gray's safety?

A Baltimore Police Academy instructor and other witnesses have spoken of "shared responsibility" for a detainee's safety, although Porter's defense lawyers have suggested ultimate responsibility lies with the driver of the police van. Whatever jurors decide, this will be a key issue not only in Porter's trial but those of the five others. Officer Caesar Goodson, the van's driver, faces the most severe charge of depraved heart second-degree murder. His trial is scheduled to begin Jan. 6.

Update, Dec. 9

In his testimony, Porter said that he couldn't tell Goodson what to do, because they were the same rank. The defense made sure the jury heard that Goodson has been serving in the Baltimore Police for 16 years, versus Porter's three.

What happened in the van?

This is the central mystery of the case. The van's internal video camera was not working, and the detainee who was briefly in the other side of the van could not see Freddie Gray. Maryland's assistant medical examiner Carol Allan told jurors that she believes Gray managed to stand up, and then — unable to brace himself since he was handcuffed and in leg shackles — was thrust forward by the van's movement, breaking his neck and severely pinching his spinal cord. Allan and a neurosurgeon the prosecution brought in as a witness both dismissed the defense's suggestion that Gray could have injured himself, saying the force of the impact was too strong. But defense attorneys did get Allan to concede that no one described seeing Gray stand up, and there is no physical evidence he ever did so.

The trial is building toward the closing arguments, and Judge Barry Williams has said the trial should end by Dec. 17.

Update, Dec. 9

The defense called Dr. Vincent Di Maio, an expert in forensic pathology, to give his own interpretation of Gray's autopsy. In his view, Gray suffered an immediate catastrophic neck fracture, in which case the injury could not have happened until near the end of the van ride, when Gray was found unconscious. Di Maio said Gray's death should have been ruled an accident, not a homicide.

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