The passenger walked away holding a gun and refused to drop it when ordered, leading the police officers to open fire, wounding the suspect. The man was taken to a local hospital in critical condition and later died.
Dotson added that the driver, also 19, cooperated with the police and is now in custody.
According to St. Louis media reports, both teenagers in the car were black. The officers involved in the incident both served in the local police department for about eight years. One of them is black and the other is white.
St. Louis Post-Dispatch said protesters started gathering at the scene an hour after the shooting.
St. Louis County, Missouri witnessed lasting mass protests after the killing of unarmed 18-year-old black teenager Michael Brown by a white police officer in August 2014. The killing sparked a string of nationwide demonstrations against police brutality and racial profiling.