Danny Takemoto, 46, of Benicia, was arrested on suspicion of felony involuntary manslaughter, said Concord police Lt. David Chilimidos.
Police estimated the child had been in the vehicle for six to seven hours.
Firefighters and police responded at 3:50 p.m. to a parking lot at a Siemens AG division at 4040 Nelson Ave., Chilimidos said.
Takemoto told police he was supposed to drop off the child at a day care center and forgot, instead driving to work, Chilimidos said.
Takemoto emerged from his workplace and found his son dead. Distraught, he asked others to call 911. The baby sat in a rear-facing child safety seat in the back seat of a blue, late-model Honda Odyssey, which had tinted windows that appeared to be rolled up. It was parked in front of one of the Siemens buildings.
The child's mother arrived at the scene later. One of the two was taken to John Muir Medical Center in Concord by an American Medical Response ambulance.
The temperature in Concord reached 81 degrees Wednesday, but would have been much higher in an enclosed vehicle. Heatstroke can occur when the body's core temperature reaches 104 degrees. A core temperature of 107 degrees is considered lethal, according to the Department of Geosciences at San Francisco State University. A child's body can warm three to five times faster than an adult's.
The investigation will include reconstructing the hours leading up to the baby's death. "We're trying to find out how this tragedy happened," said Concord police Sgt. Michael Finney. "We have a lot of people to talk to," Chilimidos said.
Bishop Robert Green of the Contra Costa sheriff's Chaplains Program arrived to help police and firefighters, as well as the family and others traumatized by the incident.
An autopsy likely will be conducted Thursday, Finney said.
While horrifying, such accidents are extremely rare, says Rona Renner, an East Bay parent educator and host of the call-in radio show "Childhood Matters."
"I don't think there are any shoulds here," Renner said. "It's a terrible tragedy and we can all feel grief for the family. Losing a child is the worst thing that can happen to a parent."
Still, such cases offer parents a prompt to reflect on their own lives.
"It's a time to take a deep breath and ask yourself what's your stress level like? Do you feel sometimes like you're going to not be able to carry through with that utmost responsibility, which is caring for your kids? If you're getting close, reach out for some help."
"Sleep-deprived, hard-working, stressed parents are not functioning at their best," said Renner. "Not to make any excuses, but there but for the grace of God go I."
"All parents can find a time in their life when they look back and find something they wouldn't do again, but (those times) usually aren't deadly."
At least 15 small children have died so far this year nationwide after being left in hot vehicles, according to a study published in Pediatrics, the Journal of the American Academy of Pediatrics and updated July 12. Last year, 29 children died, and 42 died in 2005, and a total of 336 have perished since 1998. Of the total, 101 were less than a year old.
Under state law, leaving a child 6 or younger alone in a vehicle is a citable offense. A San Jose father was convicted of felony involuntary manslaughter after his infant son died in a hot car in 2001 and was sentenced to community service and probation.