The shooting happened at about 8:20 a.m. in the area of Somersville Road and Delta Fair Boulevard.
The victim and his passenger told police that they had been stopped at a red light on Somersville Road at Delta Fair Boulevard when a white, four-door sedan pulled up behind them, police said.
When the light turned green, the white car went around them on the left and then both vehicles made a right turn onto Delta Fair Boulevard.
As they were making the turn, the driver of the white car fired a shot at the victim. The bullet went through the driver's side door and struck the victim in the back.
The white car, which contained a male driver and a female passenger, then accelerated west on Delta Fair Boulevard.
The victim called police and was taken to a hospital, where he remains in critical but stable condition today, police said.
While investigating the case, police reviewed footage from surveillance cameras near the scene and identified a car captured by several cameras.
On Friday, a patrol officer spotted a car matching the description of that vehicle near where the shooting happened. The officer pulled the car over and contacted Cornelius Brown, a 24-year-old resident of Antioch and San Francisco, police said.
The officer found a loaded gun in the car.
During a subsequent search of Brown's home, police also allegedly found a rifle, ammunition and marijuana packaged for sale, police said.
According to police, Brown admitted to the shooting and was arrested and booked into county jail.
Anyone with further information about the case is asked to call Detective Mellone at (925) 779-6930.