Baker had been with several friends at Johnny B's sports bar at 2500 Sycamore Drive in Antioch until the bar closed at 2 a.m., Pittsburg police Lt. Brian Addington said.
After leaving the bar, Baker and his friends reportedly got into a confrontation with at least one person in the parking lot.
Baker and his friends then left in two cars, a BMW and a Mercedes, Addington said. The other group left in a small silver SUV.
As they were driving west on Highway 4, the people in Baker's car, the BMW, noticed that the SUV was following them.
Baker, who was driving, sped up to get away from the SUV, but it sped up as well, Addington said.
The SUV then pulled up alongside the driver's side of the BMW and someone inside shot into the car, striking Baker.
Once hit, Baker swerved off the road and crashed into a guardrail.
His friends in the Mercedes stopped, put him in their car and drove him to Sutter Delta Medical Center in Antioch, where he died.
The hospital notified Antioch police at about 2:20 a.m. A stretch of westbound Highway 4 was closed until about 10:30 a.m. Saturday while police collected evidence from the crime scene, Addington said.
Investigators are still trying to determine what the motive for the shooting was and what the confrontation in the parking lot was about.
Baker had recently completed his federal parole. He had prior convictions for drug distribution and felony gun possession, Addington said.