"He was charged today and that is the beginning and we know we will get justice," Nina Taylor, the victim's relative, said.
LaRoyce Tankson, 31, turned himself in to law enforcement authorities late Thursday. Bond was set at $250,000 for Tankson on Friday.
"I think it is ridiculous they let him out for so little money and they murdered my son," Leroy Taylor, victim's father, said.
Chicago police spokesman Anthony Guglielmi said 25-year-old Chad Robertson died Wednesday.
Robertson was on a stopover in Chicago, waiting to board a bus to Minneapolis near Union Station on Wednesday, Feb. 7. His family said he was approached and shot for no apparent reason. The officer shot him in the left shoulder. He was taken to Stroger Hospital in critical condition.
Guglielmi says the man's wounds were considered non-life threatening at the time. Officers found cash and drugs on Robertson at the scene, police said, but they did not find any kind of weapon.
Cook County State's Attorney Kim Foxx told ABC 7's Judy Hsu what happened next.
"The young man took off running, got a fairly significant away from the officer. According to witness statements, the officer took a crouching stand and shot this young man in the back," Foxx said.
Tankson's attorney told a judge on Friday, the 31-year-old officer feared for his life after Robertson turned toward him and pulled something from his pocket. Foxx said that is not what witnesses saw.
"The young man did not have a weapon, did not make any moves to suggest he had put the officer's life or life of his partner or anyone else in danger," Foxx said.
Until they get justice, Robertson's family said they plan to be at all of Tankson's court appearances.
Before becoming an Amtrak police officer almost two years, Tankson was a locomotive engineer.
The Amtrak police union disagreed with Foxx's decision to charge the cop before a complete investigation was conducted.
Robertson was the father of two young children and worked construction in Minnesota. His family said he went to Union Station the night he was shot because it was cold and he wanted to warm up.
Amtrak issued a statement, saying, "Amtrak is deeply saddened to learn of the passing of Chad Robertson. Amtrak is cooperating fully with the Chicago Police Department and State's Attorney's office as they conduct an independent investigation. As there is a pending investigation, we are unable to comment further."
Attorney Douglas Hopson of Chicago told the Minneapolis Star-Tribune that Robertson's family will now be considering a wrongful death lawsuit against the officers involved.
The Associated Press contributed to this report.