The county previously offered $10,000 for information in the January killing of Tioni Theus, while the state and city of Los Angeles each put up $50,000.
The Board of Supervisors voted unanimously Tuesday to bump up its portion of the reward to $20,000, bringing the total to $120,000.
Tioni's body was discovered Jan. 8 on the side of the freeway on the Manchester Avenue on-ramp near South Figueroa Street. She was last seen Jan. 7 after telling a family member she was going to meet a friend to go to a party, officials said.
According to Los Angeles County Supervisor Holly Mitchell's motion to extend and increase the county's reward offer, Tioni "lived in Compton and was a student at Centennial High School. She was reportedly living with her father as her mother recovered from a serious hit-and-run accident. Family members say she was a straight-A student and enjoyed dance and golf.
"Investigators are urging anyone with information about this incident to contact the California Highway Patrol. A reward will encourage members of the public to come forward with information that can help identify the individual(s) responsible for her death.''
Speaking with community activists at a news conference earlier this year, Rashida Kincy, a cousin of Tioni, described her as a "vibrant young lady that was just cut from so much that was ahead of her.''
"This has been a tragedy to my family, to the community,'' she said.
The CHP asked anyone with information about her killing to call the agency at (323)644-9557. Anonymous tips can also be made to Crime Stoppers at (800)222-TIPS.