They followed the car to an H-E-B on F.M. 2920 in north Harris County.
Robert Neinast says he saw the car rear-end a truck. He said the driver sped away even though the car's hood had popped open and smoke was coming from the engine.
Neinast and several other drivers chased the car to the grocery store.
Police eventually arrested three people in the car.
Neinast, who was once a volunteer firefighter, says he felt he had to help the public.
"They were also endangering other people going down 2920," Neinast said. "People were getting out of their way because, like I said, the hood was up.The guy was looking through about that much of the windshield trying to drive."
Authorities suspect the two men and one woman in the car were involved in the robberies of a Radio Shack and that same H-E-B store earlier in the day.