A man, a woman and two young children were found shot to death on Friday alongside the Florida Turnpike about 100 miles north of Miami, police said.
The woman was clutching the children as if to protect them, St. Lucie County Sheriff Ken Mascara said. "This is definitely not a murder-suicide. This is a murder," he said.
The identities of the victims were not released.
Sherriff Mascara said the four victims appeared to have been travelling southbound on the Turnpike with a group in a large vehicle, perhaps a sport utility vehicle or van.
Tyre tracks at the scene indicated the vehicle pulled off the highway in Port St Lucie, stopped at the location where the bodies were found in a grassy area alongside the road, then continued southbound, he said.
"The woman, in a defensive posture, had both of the children surrounded underneath her arms in an effort that we can assume was to protect them from the gunfire," Sheriff Mascara said.
The victims were either kneeling or lying on the ground and each suffered multiple gunshot wounds, he said.
The male victim was 29 years old, the woman 25 and the children were believed to be 4 and 6 years old, he said.
A roadside surveillance camera nearby was not turned on at the time.