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Murder charges were leveled Friday against an Amtrak police officer accused of fatally shooting an unarmed Minneapolis man during an encounter in downtown Chicago.

LaRoyce Tankson, 31, made a brief appearance Friday before Cook County Judge Maria Kuriakos Ciesil, who set his bail at $250,000. He had turned himself in to police the night before, prosecutors said.

Tankson was charged with a single count of first-degree murder in the shooting of 25-year-old Chad Robertson, following a joint investigation by the FBI, Cook County State's Attorney's Office, and the Chicago Police Department's Internal Affairs Division, prosecutors said. The charge carries a maximum sentence of 25 years to life in prison if Tankson is convicted.

Robertson died Wednesday morning at the Chicago hospital where he had been treated for a gunshot wound from the Feb. 8 incident.

Prosecutors say that he was shot while running from Tankson and his partner, who had stopped Robertson and two of his friends apparently after catching them smoking marijuana outside Union Station, Chicago's main transit hub.

An autopsy revealed that the bullet pierced Robertson's left shoulder and lodged in his neck, prosecutors said.

Chicago police officials said last week that they did not recover any weapons on Robertson, although they did find a small amount of drugs and cash at the scene.

The day before Robertson died, his family filed a federal lawsuit alleging excessive force and a violation of his Fourth Amendment right against unlawful search. Douglas Hopson, a Chicago-based attorney working with Robertson's family, said that he intends to bring a wrongful-death suit against Tankson and the train agency.

Cook County State's Attorney Kim Foxx, whose office is charging the case, took over as the county's top prosecutor last fall after her predecessor drew sharp criticism for her prosecutorial decisions in several high-profile police shootings, including that of 17-year-old Laquan McDonald, who was shot 16 times in October 2014. More than a year passed before officer Jason Van Dyke was charged with first-degree murder in the shooting. He awaits trial.

Foxx has now charged two police officers with murder since taking office three months ago. A spokeswoman for the office declined further comment on Friday afternoon.

On Friday, Robertson's family and friends expressed cautious relief over prosecutors' decision to charge Tankson.

"I feel weirdly optimistic that we will get justice," said Chad Robertson's sister, Nina Robertson.

The family was "broken" by his death, said Nina Robertson, who has marched in several protests following other high-profile police shootings of black men. "And I never in a billion years thought that it could happen to me or my family."

She described her brother as an outgoing man who made friends easily, and a gifted drawer. A native of the North Side, he had grown up playing football at Farview Park, where their mother had worked for years. He left behind two young children.

She said that when the family first arrived at the hospital, doctors gave them a frank diagnosis: that there was a possibility that he would never regain feeling in his arms and legs. At the time, his wound was still considered nonlife-threatening, she said. "Once I saw he was alive, I thought, 'Oh, it's only up from here,' " she said.

Train station encounter

Family members said they pieced together Robertson's final moments after speaking with his two travel companions, at least one of whom was later called to testify before a grand jury weighing criminal charges against the officers, Hopson said.

Robertson, they said, had been traveling back home from Memphis, where he and a friend had attended a funeral, when their bus made a scheduled hourlong stopover in Chicago. He and his companions reportedly ducked into Union Station about 8 p.m. to escape the cold weather, before going back outside to smoke some pot, authorities said.

They were approached about half an hour later by Tankson and his police partner, who hasn't been identified.

The officer told Robertson to put out the "cannabis cigarette," authorities said, which he did, before apologizing and telling Tankson that he wasn't from Chicago. Robertson and one of his companions then started walking south on Canal Street to a nearby restaurant, while the third member of their group went back to the transit station to retrieve a bag, according to court filings. The officers again approached them, the filings said.

Tankson "did not suspect Robertson [or his companions] of any criminal offense other than possession of cannabis nor did they witness them committing any other criminal offense," prosecutors said.

Nevertheless, Tankson and his partner started to frisk the pair.

Relatives said Robertson panicked, pulled away from the officer and took off running, heading southeast on Canal Street when the shooting occurred. Tankson reportedly drew his department-issued Glock .40-caliber handgun, dropped to one knee and fired a single shot at Robertson, who by then was about 75 to 100 feet away, authorities said.

The shot dropped Robertson in the middle of the street, where he lay motionless, they said.

At least six people witnessed the incident, none of whom saw Robertson "gesture or turn" toward the officers as he fled, according to authorities.

Union supports officer

The union that represents the train agency's roughly 500 officers said that it is "standing in full support" of Tankson, who has been with the force for nearly two years.

"While we are confident that any judge or jury will find that Officer Tankson acted within the law and out of fear for his life, we disagree with the State's Attorney's decision to charge against Officer Tankson before a complete and through investigation was conducted," the Amtrak Police Fraternal Order of Police said in a statement.

A union spokeswoman said that Tankson was cooperating with the investigation. A message for comment wasn't immediately returned on Friday afternoon by Tankson's attorney, William Fahy, who has represented several police officers.

Tankson was hired in September 2015, having previously worked for Amtrak as a train conductor. He has been assigned to Chicago, a major hub, his entire career.

Among his responsibilities as a patrol officer were to assist "passengers with their concerns and counter-terrorism," union president David Pearlson said Friday. He said Tankson has earned high marks from his supervisors.

Pearlson said that all of the agency's officers have undergone use-of-force training in the past four months.

He declined to comment on the incident, citing the ongoing investigation.

Libor Jany • 612-673-4064