ST. PAUL — A Minnesota police officer, whose fatal shooting of a black motorist transfixed the nation when his girlfriend livestreamed the aftermath, was acquitted of all charges Friday.

The officer, Jeronimo Yanez, had been charged with second-degree manslaughter and endangering safety by discharging a firearm in the shooting of Philando Castile.

After the verdict, jurors and Yanez were quickly led out of the courtroom, and Castile’s family left immediately. When a deputy tried to stop his mother, Valerie, she yelled “Let me go.’’

Later, she said: “My son loved this city, and this city killed my son. And a murderer gets away. Are you kidding me right now?’’

She continued: “The system in this country continues to fail black people and will continue to fail us.’’

A handful of protesters gathered outside the Ramsey County courthouse after the verdict. “It’s not us that were on trial, it was the system that was on trial,’’ Mel Reeves, a community activist, said. “Yanez worked for the system. He killed somebody, right. Philando Castile got victimized by the system.’’

The verdict, which came on the fifth day of deliberations, followed the acquittal last month of an Oklahoma police officer who killed an unarmed black man and, of particular interest in Minnesota, a prosecutor’s decision last year not to charge the Minneapolis officers who killed another black man, Jamar Clark.

Castile’s death on July 6, 2016, became international news almost immediately because his girlfriend, Diamond Reynolds, who was sitting in the front seat, streamed the aftermath live on Facebook.

From that moment, the case hinged largely on conflicting accounts of what Castile was doing before he was shot. Yanez claimed in the Facebook video, and again last week on the witness stand, that he believed Castile was grabbing a gun.

Reynolds said in the video, and testified in court, that Castile had been trying to cooperate and produce his driver’s license.

The killing led to weeks of protests here, including an encampment outside the Minnesota governor’s mansion, and renewed questions about how officers use force and treat black people.

Prosecutors said Yanez had created a dangerous situation, perceived a threat where none existed and, in addition to killing Castile, almost wounded Reynolds and her young daughter in the back seat.

“He was making assumptions and jumping to conclusions without engaging in the dialogue he was trained to have in a citizen encounter like this,’’ Jeffrey Paulsen, a prosecutor, said in closing arguments Monday. “And that’s his fault, not the fault of Philando Castile.’’

Castile was licensed to carry a gun and was recorded on a dashboard camera video calmly telling Yanez that he had a weapon in the car. Yanez told him not to reach for the weapon, and Castile and Reynolds both tried to assure the officer that he was not doing so.

Within seconds, Yanez fired seven shots. Prosecutors said Castile had mentioned his gun to allay concerns, not to threaten the officer or escalate the situation.

“If someone were just about to reach in their pocket and pull out a gun and shoot an officer, that’s the last thing they would say,’’ Paulsen said.

Earl Gray, a lawyer for Yanez, said his client had to react quickly to what he believed was an imminent threat. Gray said Yanez smelled marijuana, believed Castile matched the description of a recent robbery suspect, and saw him grabbing a gun.

“We have him ignoring his commands. He’s got a gun. He might be the robber. He’s got marijuana in his car,’’ Gray told jurors. “Those are the things in Officer Yanez’s head.’’

Yanez did not tell Castile about the robbery suspicions, only that his brake light was out. But Gray said that this approach made sense, and that Yanez had acted reasonably given his training and what he knew that night.

“He did what he had to do,’’ Gray said, adding that the situation was “tragic.’’

The jury of 12, including two black people, had to sort through the competing narratives. Both prosecutors and defense lawyers said the video footage supported their version of events.

At Yanez’s trial, in a small courtroom in downtown St. Paul, defense lawyers made repeated mention of Castile’s and Reynolds’s use of marijuana. The drug was found in Castile’s car after the shooting, and Gray claimed that Castile had been under the influence of marijuana and delayed in his reactions at the time of the shooting.

But Paulsen, the prosecutor, said that version of events was contradicted by video. He said footage showed that Castile was driving normally, pulled over quickly, and was alert and courteous when talking to Yanez. He accused the defense of blaming the victim.