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Three people including a three-year-old child suffered serious injuries in the crash on Thursday afternoon on the A10 at Littleport.
Officers attended the incident after being called at about 4.15pm and closed the road between Ely and Littleport.
As a result of the collision three people, a woman in her 30s, a man in his 50s and a three-year-old child, sustained serious but not life threatening injuries and were taken to Addenbrooke's Hospital.
PC Chris Boother said: "We are keen to hear from anybody who was in the area, specifically drivers with dash cam footage, traveling in either direction just before the incident.
"We are also keen to speak to the driver of a 4x4 seen in the area just before the collision."
Anyone who may have seen the collision is asked to contact the road policing unit on 101 or Crimestoppers, anonymously, on 0800 555 111, or via their online reporting system at www.crimestoppers-uk.org