A witness alerted Florida Highway Patrol troopers to the bodies of a man, woman, boy and girl on the southbound shoulder south of the Route Midway Road in Port St. Lucie around 7:45 a.m., according to the St. Lucie County Sheriff's Office. They had been shot multiple times, Sheriff Ken Mascara said.
The shootings occurred sometime after midnight at the scene, officials said.
"It appears to be a family," Mascara told CBS News correspondent Peter King. "It's a male and female between the ages of 25 and 35, and then two children, which I would estimate between 4 and 6 or 7."
Authorities have determined their identities but were not releasing that information yet.
The woman was found holding the children.
"The mother, in a defensive posture, attempted to protect the children from the shooting, and as such, got herself clutched around both children," Mascara said.
No vehicle was found near the bodies, but there were tire tracks from a vehicle that apparently pulled back from the scene and sped back onto the turnpike, Mascara said.
"This group, traveling with another group, possibly in the same vehicle, and they pulled over and executed them right there along the shoulder of the roadway," he said.
Investigators painted a white grid on the ground at the scene, and dug through the grass searching for evidence.
The bodies were spotted in a grassy area near the St. James Golf Club, several miles away from the nearest rest stop, the sheriff's office said.
Tracy Maynor, staff clerk at the golf club, said a resident of the homes along the golf course reported hearing "some shots or fireworks at about three in the morning. That's about all we heard."
Cameras posted along the turnpike were not recording at the time, Mascara said.
"The turnpike on occasion gets very, very desolate and dark," Mascara said.