Authorities say 19-year-old William Mack Sapp of south St. Louis County is charged with second-degree assault and leaving the scene of an accident. He is jailed on $500,000 bond following his arrest on Wednesday. It wasn't immediately clear if Sapp had an attorney.
The other alleged racer, 19-year-old Trenton Pinckard of Glen Carbon, Ill., was charged earlier this week with leaving the scene of an accident.
Police say the victim was acting as a "flag girl" to signal the start of an illegal race Monday night. One vehicle struck the girl, causing her to fall in front of the other. Both drivers allegedly fled the scene.