A second woman has died following the six-car collision on the M1 on Monday, September 15.
The 90-year-old woman died in hospital, on Tuesday, September 16, from her injuries, a 36-year-old woman died at the scene.
Police are looking for the drivers of a red car, who may have been travelling from Hertfordshire to Northamptonshire, and a white van involved in the incident.
The collision, involving six cars and a motorbike, occurred at 5.40am on the M1 Northbound close to junctions 12, closing the motorway for several hours.
PC Andy Ralph of the BCH Roads Policing Unit said “It was still dark when the incident occurred, conditions on the motorway were slightly misty and foggy and the road was damp. The 53 year old driver of the Audi and two other women passengers, aged between 89 and 90 years old, suffered a variety of serious injuries to their ribs, abdomen and spines. It appears that one vehicle moved suddenly in front of the motorcycle which caused the rider to lose control and fall from her machine. Other vehicles following behind attempted to avoid the rider and her motorbike causing several collisions between themselves.” If you have any information, call 101.