A section of Highway 39 was closed for much of the day Tuesday, after a massive collision involving nearly a dozen vehicles forced police to shut it down.

Thorsby RCMP said officers were called to the multi-vehicle collision on Highway 39 at Range Road 11 in Leduc County on Tuesday at about 9 a.m.

When police arrived, they found three tractor-trailer units, and eight personal vehicles – identified as trucks and cars, were involved in the collision.

Investigators have since found that one semi-truck had rear-ended another truck into a number of vehicles, including another semi-truck.

Police said later Tuesday that six people were injured in the crash, one of those was in critical condition, another in serious condition and the remaining four suffered minor injuries.

RCMP said at least one individual was extracted from a damaged vehicle before being taken to hospital.

Alberta Health Services paramedics, Leduc County Fire Emergency Services units from Devon, Warburg, Calmar and Thorsby and one STARS Air Ambulance unit was called to the scene, in addition to officers from Leduc RCMP.

The affected section of highway was closed starting late Tuesday morning, and RCMP said it was expected to stay closed until at least 6:30 p.m. Tuesday.

RCMP are still investigating the crash, but said heavy fog and smoke in the air at the time of the crash limited visibility in the area – an area resident was carrying out a controlled burn of a field in the area at the time.

Plus, RCMP said there was an earlier, less serious motor vehicle collision involving two cars about an hour and a half before the major crash.