Four members of the same family - including three children under the age of 10, have died in a crash on the A40 between Oxford and Cassington.
Emergency services were called just before 10pm on Monday night to reports of a collision involving a people carrier and a heavy goods vehicle.
Named locally as Joshua and Zoe Powell, the couple and their children - from Chinnor near Thame, were travelling along the road close to a railway overbridge when their people carrier collided with a lorry.
The HGV driver, a 56-year-old man, suffered minor injuries.
A 30-year-old man and an 18-month-old girl have been taken to hospital and both remain in a critical condition.
Thames Valley Police remain at the scene while investigations continue to be carried out.
We are in the very early stages of our investigation into this incredibly tragic incident that has resulted in the deaths of four people from the same family. We will leave no stone unturned to ascertain what has caused this tragedy.
Officers are appealing for witnesses and anyone with dash-cam footage of the incident to come forward.
Chief Inspector Scott Long, Thames Valley Police
The family lived in the small village of Chinnor near Thame. Today members of the local community laid flowers as a mark of respect.
Rev Dr Jacky Barr, St Andrew's Church, Chinnor
The MP for the area where the accident happened has thanked the emergency services.
Robert Courts MP, Witney and West Oxfordshire, Con