Sgt. Nathan Taylor, 30, was shot to death while he was sitting in his car in front of his brother's house on April 21. He was attending classes at Antelope Valley College.
"We're always going to be hurt. We're always going to be empty. There's always going to be a piece of us missing," said the victim's sister Kinnzie Taylor.
Taylor said having someone held accountable for the murder will bring some closure.
Taylor served seven years in the Army and deployed three times to Iraq and Afghanistan. He leaves behind a fiancée and a 9-year-old daughter.
A motive has not yet been determined. Family members say Taylor was an innocent victim of gang violence. They said he must have been mistaken for someone else.
"The area where this incident occurred is an area where gang activity does occur," said Los Angeles County Sheriff's Lt. Holly Francisco. "The sheriff's department is unable to rule out if this is gang related at this time."
The reward was offered by the Los Angeles County Board of Supervisors for information that leads to an arrest and conviction. Anyone with information about the crime was urged to call the Los Angeles County Sheriff's Homicide Bureau at (323) 890-5500.
City News Service contributed to this report.