Police killed a 27-year-old man as he kicked, punched and stomped a toddler to death despite other people's attempts to stop him on a dark, country road, authorities said.

Investigators on Sunday were trying to establish the relationship between the suspect and the child they say he killed Saturday night. The Stanislaus County coroner said the boy appeared to be between 1 and 2 years old based on his size, according to county sheriff's deputy Royjindar Singh.

"It's been a long night of wondering, 'Why?' — not only for the officers and the passers-by who stopped and tried to help out, but for anyone. Why would somebody do this?" Singh said.

Singh said the coroner does not plan to confirm the identities of the suspect and victim until Monday. Because his injuries were so severe, the child will have to be identified through a blood or DNA test, he said.

The suspect had a child's car seat in the back of his four-door pickup truck. The truck caught the attention of an elderly couple at 10:13 p.m. Saturday because it was stopped in the two-lane road facing the wrong direction, Singh said.

As they got closer, the couple saw the man brutally beating the toddler behind his truck and throwing the child on the ground, according to Singh. Two or three other cars stopped, an unusual number to be passing through the remote area surrounded by a dairy, a cow pasture, a cornfield and a farmhouse, he said.

"What we got from witnesses is he was punching, slapping, kicking, stomping, shaking," Singh said. "They tried to intervene and get involved, but their efforts really didn't have an effect. The suspect was engaged in what he was doing. He just pushed them off and went back to it."

A sheriff's helicopter responding to emergency calls from the area landed in a cow pasture at 10:19 p.m. carrying a Modesto police officer who shot the man to death after he refused an order to stop beating the child, Singh said.

Paramedics tried to resuscitate the toddler, who was not breathing when they arrived. The boy was taken to a local hospital, where he was pronounced dead.

No children within the dead boy's age range have been reported kidnapped or missing in Stanislaus County, Singh said.

The incident happened on Bradbury Road about 10 miles west of Turlock, a city located about halfway between Sacramento and Fresno.