Eight people were injured in a 12-vehicle crash on the A1 near Wyboston in Bedfordshire.
The smash involved 10 cars and two HGVs on the southbound carriageway between the A428 exit slip road and the Black Cat roundabout.
It happened just after 5.30pm on Monday and the road was closed for several hours.
Paramedics were called to the scene and treated people for minor injuries.
Police are now appealing for any witnesses to come forward.
Sgt Stephen Andrews from the Bedfordshire, Cambridgeshire and Hertfordshire road policing unit, said: "We’re looking to speak to anyone who witnessed this collision to help us establish what happened.
"We are hoping to identify and speak to a lady who was travelling behind one of the HGVs.
"Although we don’t believe she was involved in the incident, we believe she may be a key witness for our investigation and were unable to obtain her details before she left the scene."
Anyone who witnessed the incident is asked to contact police on 101 or online at beds.police.uk quoting reference number 292 of 16 January.
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