A woman has tentatively identified the body as that of her 3-month-old son, and authorities were searching for the baby's father, the last person seen with him.
The child's death was ruled a homicide by blunt force head trauma, said John Florence, chief investigator for the Galveston County Medical Examiner's Office.
"The (medical examiner) is telling us it happened before the body had been tossed out in the car seat," said Galveston Police spokesman Lt. Jorge Trevino.
The autopsy did not offer additional details about how the infant was killed or give an exact time of death. A couple looking for wildlife Tuesday morning found the body, clad only in a diaper, in a grassy area of Galveston, an island city about 50 miles southeast of Houston.
There had been initial reports that the infant might have been thrown out of a moving vehicle.
"But the location of the child and the baby seat is not consistent with a moving vehicle," Trevino said. "It was likely the car was stopped."
The infant was in the car seat when it was tossed, and the child was apparently thrown out when the seat hit the ground, Trevino said.
Trevino said investigators have learned that the couple thought to be the infant's parents had been arguing Monday night in their home in Alvin, about 30 miles northwest of Galveston.
By the time the child's father left with his son early Tuesday to get a car repaired, the couple were no longer arguing, Trevino said. The mother began calling hospitals hours later after becoming concerned about the baby because the father had left with few provisions for the child.
The infant's father is being called "a person of interest" in the investigation, police said.
The infant has yet to be positively identified. Investigators may be able to match the body with hospital footprint records; if not, they will have to rely on DNA testing, which would take several days, Trevino said.
The discovery of the baby's body comes just three months after a fisherman found the remains of a 2-year-old girl in a plastic box that had washed ashore in Galveston Bay. That child, dubbed "Baby Grace," was later identified as Riley Ann Sawyers of Spring.
Riley's mother and stepfather are charged with capital murder in her beating death.
