The incident happened just north of FM 1960 around 3 a.m. Tuesday, according to a lieutenant with the agency.
Investigators said they believe multiple drivers could have hit the individual, though the lieutenant said none of them stayed on scene.
It is unknown if the victim, who hasn't been identified, was trying to cross the freeway or walking along the shoulder.
The sheriff's office told ABC13 the freeway would remain closed for hours as the investigation continued, but it has since been reopened.
Live traffic map