In the Berkeley County accident, the toddler was riding on the gasoline tank of a four-wheel ATV when the driver went up an embankment and the vehicle flipped over, said Trooper Z.J. Vaughan.
Steven died of a severe head injury, according to Berkeley County Coroner Sandra Brining.
Vaughan said Monday he had not been able to interview the boy's parents, Jason Ward and Stephanie LeBre of Inwood, neither of whom were at the scene of the accident.
Steven was riding with Domingo Hernandez, of Glengary, W.Va., in a remote wooded area near Glengary on Saturday, Vaughan said.
Hernandez, a friend of a family member, and others had gathered at the area for a weekend outing, Vaughan said.
Hernandez had been giving rides to the children on the ATV and had gone up the embankment successfully earlier, Vaughan said.
Hernandez is cooperating with investigators, Vaughan said. Vaughan said he will meet with the county prosecutor to determine if any charges will be filed.
The accident occurred on Saturday evening and the boy was taken to City Hospital in Martinsburg, W.Va., by private vehicle.
Attempts to revive the boy were unsuccessful and he was pronounced dead at the hospital.
State police did not learn of the death until Sunday morning.
The scene of the accident is a rugged area accessible only by foot or four-wheel drive vehicle, Vaughan said.
Five people have been killed in ATV accidents in West Virginia this year, state police said. Six people died and 37 people were seriously injured in ATV accidents in West Virginia in 1996.
According to the ATV Safety Institute, more than 70,000 West Virginians own ATVs.
"Because it is such a growing recreational activity we cannot emphasize enough the importance of operating them safely and responsibly," said West Virginia State Police Superintendent Col. Gary Edgell in a news release.