(CNN) Baltimore police Officer William Porter, one of six officers charged in the April death of Freddie Gray , testified Wednesday that he didn't realize Gray was injured until the final stop of the van ride in which the prisoner suffered a broken neck.

There was mucous around Gray's nose and mouth, Porter testified. He and another officer tried to assist Gray. They placed Gray in a "lifesaving position" and waited what "felt like an eternity" for a medic to arrive, Porter told the jury.

He is the first of the six officers to be tried.

With one of his attorneys at times demonstrating Gray's position, Porter testified that he never believed Gray needed immediate medical attention until the sixth and final stop.

"Are you sorry Freddie Gray died?" one of his lawyers asked.

"Absolutely," said Porter, who had testified that he knew Gray from the West Baltimore neighborhood he patrolled.

"Freddie Gray and I weren't friends, but we had a mutual respect," Porter said.

He added, "Any kind of loss of life, I'm sorry to see that."

Earlier, Porter testified that he did not call for a medic before that final stop because Gray had not exhibited signs of a medical emergency.

"I didn't see anything externally, cuts or wounds," Porter said.

Asked whether Gray said he couldn't breathe, Porter replied: "Absolutely not."

Porter's four hours on the stand Wednesday came a day after the prosecution rested a weeklong case Tuesday in the death of Gray, who authorities said broke his neck while being transported in a police van, shackled but not wearing a seat belt.

Porter has pleaded not guilty to charges of involuntary manslaughter, second-degree assault, misconduct in office and reckless endangerment.

'He did nothing'

Prosecutors allege that Porter, summoned by the driver to check on Gray during a stop on the way to a police station, did not immediately call for a medic when Gray asked for help. They also allege that Porter, after seeing that Gray was not buckled in, did not get him into a seat belt as was department policy.

"This defendant did nothing to get him a medic or get him to the hospital. He did nothing when he could have saved a man's life," State's Attorney Michael Schatzow said in court Tuesday.

Gray's April 12 arrest in Baltimore was captured in bystander videos. Prosecutors said Porter, 26, was present for all but one of six stops on Gray's 45-minute ride to the Western Division police station.

Gray's death a week after the injury sparked outrage and demonstrations, some of which were plagued by arson, vandalism and looting despite his family's pleas for peace.

Mayor Stephanie Rawlings-Blake joined Police Commissioner Kevin Davis at a news conference Wednesday to ask the public to respect the judicial process.

"Whether you agree or whether you disagree with the jury's ultimate verdict, our reaction needs to be one of respect in Baltimore's neighborhoods, and the residents and businesses who make up our city should reflect that respect," she said.

On the stand, Porter said the only time he heard a man say he couldn't breathe that day was when Gray was initially arrested. He said he did not know it was Gray at the time.

Porter said he saw the police wagon shaking side-to-side and heard kicking during the second stop.

On the fourth stop, Porter said, the wagon's driver asked him to check on Gray, who was facedown and asking for help. He helped the prisoner sit on a bench, with Gray "supporting his own head," Porter said.

Some prisoners exhibit what Porter called "jail-itis," feigning injury to avoid going straight to the lockup. Gray did not say he was having trouble breathing, Porter repeated.

The officer said he told the wagon's driver, Officer Caesar Goodson, that Gray wanted to go to a hospital. He did not see Gray again until the final stop.

Seat belts not typically used in police wagons

Porter, who joined the Police Department in 2010, testified that he used seat belts when transporting people in his cruiser but never in the tight quarters of the wagon in which Gray sustained the fatal injuries. Part of the reason, he said, was officer safety.

Of the roughly 150 prisoners he put in police wagons during his time on the job, Porter said, none was secured with a seat belt.

"It is the responsibility of the wagon driver to get the prisoner from point A to point B," he told the jury.

During his six weeks of field training, Porter testified, people transported in wagons were never secured with seat belts. At the police academy, he said, cadets were told to use seat belts while transporting prisoners, but were not shown how to do so.

Cross examination

Photos: Faces of Baltimore Photos: Faces of Baltimore Ma'lae Jones lives in Sandtown-Winchester, the same neighborhood in which Freddie Gray was raised. She is a kindergarten student at New Song Academy. "I want to be a ballet teacher" when I grow up, she said. The trial for the first of the six police officers charged in the death of Freddie Gray is due to begin Monday in Baltimore. The death of Gray while in police custody sparked some of the most violent protests the city has ever seen. It's been seven months since Gray's death. We returned to Baltimore to photograph residents as the city works to make sense of the year's events. Hide Caption 1 of 11 Photos: Faces of Baltimore Born and raised in Baltimore, Kondwani Fidel had a tough upbringing and knows what it means to lose loved ones to violence. Fidel is a substitute teacher and spoken word artist. His poems tell stories of a rough upbringing in Baltimore while attempting to inspire change. When using poetry to connect with students, "I used my story as a blueprint for them," said Fidel. Hide Caption 2 of 11 Photos: Faces of Baltimore As the new police commissioner of Baltimore, Kevin Davis came into his role in the immediate aftermath of the unrest. This is a pivotal point for the city, said Davis. "People who find themselves in the midst of a historic moment don't always realize that they are in the midst of a historic moment," he said, "I realize that's where we are." Davis is starting by establishing relationships with various groups in the community to prevent the type of unrest the city experienced in April and May. Hide Caption 3 of 11 Photos: Faces of Baltimore A 44-year veteran of Baltimore schools, Nancy Neilson, principal of New Song Academy, says there is something special about her current school. All of the students come from the immediate neighborhood in west Baltimore. Her students perform better than many of their counterparts, but unfortunately, it is not rare for them to be touched by the prevalent violence in the community. During the unrest earlier this year, "they were frightened because they didn't know what was going to happen," Neilson said. Hide Caption 4 of 11 Photos: Faces of Baltimore Niamke Nnamdi graduated from high school last year, and he is ready for the next step. "I am joining the Army," he said. After taking entry exams, Niamke qualifies for almost any position in the Army. Hide Caption 5 of 11 Photos: Faces of Baltimore "There was a sense of collective outrage about what happened to Freddie Gray that tapped into a deep well of despair," said William Murphy, the attorney for the family of Freddie Gray. He says the biggest lesson that Baltimore can take away from the year's events is "that you can only ignore a group of people for so long." Hide Caption 6 of 11 Photos: Faces of Baltimore Former mayor of Baltimore, Sheila Dixon has emerged back into the spotlight after her 2010 resignation as part of a plea agreement in a criminal case. "I am beyond that," Dixon said, "That doesn't define who I am." Dixon says she is focused on rallying behind communities, addressing the dramatic rise in homicides and providing strong leadership. Neither Freddie Gray case nor the unrest that followed prompted the campaign, Dixon said. Hide Caption 7 of 11 Photos: Faces of Baltimore A bartender at restaurant in the Canton neighborhood of Baltimore, Lincoln Kosman remembers the days after the riots. During the citywide curfew, Kosman said residents looked out for each other, and people checked on their neighbors. "It's affected the city in a good way," he said. "People have come together." While the spotlight remained on Baltimore, Kosman said, "I really hope that we show we're a good city. I hope we do ourselves proud now that everyone is watching." Hide Caption 8 of 11 Photos: Faces of Baltimore Kaleb Tshamba belongs to the historic Arch Social Club. More than 100 years old, the club has been traditionally African American and for males only. "We are one of the only historic black places left in Baltimore," Tshamba said. The club sits across the street from the CVS store that was burned during the April riot. Tshamba and other members of the club participate in the 300 Men March, which calls for a stop to violence. A sign on the front of the club's building reads, "We must stop killing each other." Hide Caption 9 of 11 Photos: Faces of Baltimore Tyler Fullwood, 13, make A's and B's in school and wants to be a lawyer when he grows up. As the oldest of three siblings, Fullwood said he sometimes fears for the safety of his family and friends. "There was a shooting behind the school and a student almost got hurt," he said. Some parts of Baltimore are dangerous, but it's not all bad, he added. Hide Caption 10 of 11 Photos: Faces of Baltimore Brittony McKenney had traveled around the world with the Navy by her mid-20s. Though she loved traveling, after six years, she said, "That was enough for me." Now McKenney is a year away from earning her bachelor's degree in broadcast journalism. "I thought coming back to Baltimore would taint my growth and the frame of mind I had," she said, "But it did the opposite. It enhanced me even more." Baltimore is rich with culture, compassion and love, McKenney said. "No matter where I go, I'll always be Baltimore." Hide Caption 11 of 11

During cross examination, chief deputy state's attorney Schatzow pressed Porter on why an investigating officer noted that Porter told her Gray said he couldn't breathe on the fourth stop. Porter said she misunderstood.

"When she said tell me what happened," Porter testified, "I started at the beginning."

Schatzow asked Porter whether a phrase the officer used -- the "stop snitching culture" -- was the reason why the officer didn't identify the cop who placed Gray in the wagon.

"I'm actually offended you would say something like that," Porter responded. "I gave all of the officers' names."

Schatzow closed his cross examination: "It's ingrained in you to protect life, but at stop four and stop five you did nothing to protect Freddie Gray's life, did you?"

"Untrue."

'Accidents happen'

It remains unclear exactly how Gray was injured, but Dr. Carol Allan, the assistant state medical examiner, testified Monday that Gray probably received his neck injury between the van's second and fourth stops.

Although there is conflicting testimony about when, witnesses said Gray complained at least once of being unable to breathe. Gray asked Porter for medical assistance when the officer checked on him at the fourth stop, according to Allan's testimony and Porter's interview with department investigators. Allan said Gray probably was injured when the van stopped suddenly.

The delay in getting Gray medical attention led Allan to classify Gray's death as a homicide.

"If he had gotten prompt medical attention, it would not have been a homicide," she stated, adding that Gray probably would have survived if van driver Caesar Goodson had rushed him straight to a hospital when he told Porter "I can't breathe."

On Wednesday, Dr. Vincent Di Maio, a forensic pathologist testifying for the defense, said Gray's death should have been declared an accident and that the fatal injuries likely occurred during later stops on the police wagon.

"It was just an accident and accidents happen," he said.

Di Maio said he believes Gray suffered a "single catastrophic event" between the fifth and sixth stops -- not between the second and fourth, as the prosecution claims -- that left him paralyzed and unable to speak.

The trial is expected to end by December 17.