A Colorado jury split its decision in the 2019 death of a young, unarmed Black man, convicting one police officer but acquitting the other on Thursday afternoon, in a case that drew national attention after the murder of George Floyd.

A mostly white jury of seven men and five women deliberated for just over two days following the three-week trial against Randy Roedema and Jason Rosenblatt, for the death of Elijah McClain. But jurors found only Mr. Roedema, 41, guilty of criminally negligent homicide and assault.

Both officers — who were tried together — had been charged with both crimes, as well as manslaughter. Mr. Roedema will be sentenced on Jan. 5.

First responders, including the police and paramedics, had placed Mr. McClain, 23, in a chokehold during a confrontation in Aurora, Colo., and injected him with a powerful sedative. He died in the hospital several days later. Three police officers and two paramedics were charged in his death, and are being tried in three separate trials. The joint trial of Mr. Roedema and Mr. Rosenblatt was the first.

The split verdict came as a shock among those who had been pushing for accountability since Mr. McClain’s death four years ago.

“How do you convict one and acquit the other? How can you call this justice?” said Candice Bailey, an Aurora activist who led many of the early marches and demanded police reform.

During the trial, Mr. McClain’s mother, Sheneen McClain, had been adamant that all five men should be convicted. “None of them did their job that night the way they were supposed to. The police didn’t do their job that night and neither did the paramedics,” she said before the trial ended,adding, “They worked as a team to murder my son.”

At the time the officers were indicted, in 2021, the Aurora Police Association called the charges an overreaction that damaged the Police Department, saying, “Our officers did nothing wrong.”

Ms. McClain had attended the trial over the weeks, enduring graphic videos and photos of her son’s death. “This is the divided states of America, and that’s what happens,” Ms. McClain told reporters after she left the courtroom on Thursday with one fist raised in the air, according to Colorado Public Radio.

Among activists and criminal justice observers, the case almost instantly became a barometer of police accountability in the post-George Floyd landscape.

It was among the first involving a death in custody and charges against the police since the summer of 2020 when Mr. Floyd was murdered by a Minneapolis cop, which provoked widespread protests and calls for reform.

Mr. McClain’s death stood apart from some other police violence cases, in part because he was not suspected of committing any crime, but also because five first responders were charged in his death after a state investigation.

The case, which deeply divided the city of Aurora, also helped to usher in a sweeping police accountability law in Colorado, among the most comprehensive in the nation. The use of ketamine is now restricted and chokeholds are banned in Colorado.

Phil Weiser, Colorado’s attorney general, said on Thursday that the verdict was about “accountability. Everyone is accountable and equal under the law.”

Mr. McClain, a massage therapist, was walking home from a convenience store in Aurora on Aug. 24, 2019, when he was stopped by the police after a 911 caller described Mr. McClain as “sketchy.” He was waving his arms, dancing and wearing a mask, which his mother said he did because he was anemic and needed to stay warm.

In an 18-minute confrontation, police officers placed him in a carotid chokehold, and then paramedics injected him with ketamine. As he was handcuffed and laying on the ground, Mr. McClain vomited, pleaded for his life and told the police, “I can’t breathe” seven times, according to prosecutors — with some of the episode captured on police body-camera footage. He eventually suffered a cardiac arrest and died in the hospital several days later.

State prosecutors relied heavily on body camera audio and video to argue that the two officers’ actions were aggressive and unnecessary. By the time paramedics injected Mr. McClain, they argued, he was already weakened by the restraints and struggling to breathe.

Mr. Rosenblatt and Mr. Roedema’s lawyers both insisted that the police were doing their jobs and shifted the blame to others: more senior officers on the scene, the paramedics who were in charge of medical treatment and Mr. McClain himself.

But Mr. Rosenblatt’s lawyer, Harvey Steinberg, made distinctions in his defense, arguing that Mr. Rosenblatt was away from Mr. McClain when he was injected with the ketamine.

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Mr. Roedema has been suspended from the force. Mr. Rosenblatt was fired in 2020 after he received photos from other Aurora police officers re-enacting the chokehold used on Mr. McClain. He had replied “Haha” to the text.

Dr. David Beuther, a nationally recognized pulmonologist, testified for the prosecution that the officers’ actions caused acidosis, hypoxia and aspiration — physiological factors that contributed to Mr. McClain’s death. Dr. Beuther said that his state was so compromised before the ketamine that he would have required hospitalization in an intensive care unit, and that it was unclear how much he would have recovered.

The defense did not offer any witness testimony, and the defendants did not testify.

Defense lawyers asserted that the officers were justified in their use of force because of Mr. McClain’s resistance, claiming that he was told to stop 34 times and that he reached for Mr. Rosenblatt’s gun.

Mr. Steinberg called his client, who with two years on the force had the least experience on the scene, a “sacrificial lamb.”

An acquittal, he said, “has to be the final chapter of this ugly, ugly story.”

Two months after Elijah McClain’s fatal encounter with Aurora police officers, a local district attorney declined to press charges after a coroner could not say exactly how Mr. McClain died.

The case was all but done until the 2020 death of Mr. Floyd, whose videotaped encounter with a Minneapolis cop inflamed much of the nation and brought the police brutality conversation to the forefront.

Within weeks of Mr. Floyd’s death, the Colorado governor named a special prosecutor to investigate the case. Eventually, a statewide grand jury handed down a 32-count indictment against the five first responders, and the cause of Mr. McClain’s death was changed to complications of ketamine administration following forcible restraint.