Bedfordshire Police is appealing for witnesses following a four-vehicle collision that took place on the A421 on Saturday, 13 January.
Emergency services were called at around 10.40pm to reports of a collision on the westbound carriageway between the A600 and A6 junctions.
The incident involved a grey BMW, silver Vauxhall, black Audi and blue BMW on the westbound carriageway between the A600 and A6 junctions.
Three people were taken to hospital with serious injuries, and one person remains in a life-threatening condition.
One man has been arrested on suspicion of causing serious injury by dangerous driving and driving while disqualified.
Detective Sergeant Shona Birkby from Bedfordshire, Cambridgeshire and Hertfordshire’s Serious Collision Investigation Unit said: “This was a very serious incident and our thoughts are with those involved.
“We are working to gather as much evidence as possible to establish the circumstances around the collision.
“We’re particularly interested to hear from anyone who was travelling along the A421 at that time and has information or dashcam footage that may support our investigation.”
Anyone with information is asked to report online at beds.police.uk/tua/tell-us-about or call 101, quoting Operation Alma.