But police think they know how the lower section of the limb came to be on the Stuart Highway near Darwin on Tuesday.

It was the result of a "vehicle versus pedestrian collision", Detective Senior Sergeant Paul Morrissey said on Wednesday.

But police still don't know where the victim is, or if the person is dead or alive, and haven't ruled out foul play.

"At this time, the owner of the leg has not been identified nor the rest of the person," Det Snr Sgt Morrissey said.

"We've got no one in hospital with those sort of injuries."

Members of the public have told police they witnessed an incident about 8pm on Monday night on the section of road where the leg was found.

Major crash investigators have also found evidence to support the conclusion a vehicle hit a pedestrian, with Det Snr Sgt Morrissey saying the injured person could have survived.

"The additional information was it was survival but would need medical treatment as soon as possible," he said.

Police previously said the remains were from "a dark-skinned human being".

A forensic pathologist determined the victim was either a teenager or an adult.

It's understood the lower section of the leg had a foot attached.

The limb was spotted by a traveller who was driving on the highway in Coolalinga, 30km south of Darwin, at about 9am on Tuesday.

A crime scene was declared on the highway linking Darwin to Alice Springs and Adelaide around 10.30am.

An extensive search of the area by officers, drones, police on motorbikes and the dog squad did not find the victim or any other body parts.

Police are examining missing person reports, dashcam footage and CCTV from the area.

Australian Associated Press