Six men and two women were killed when their minibus and two lorries crashed on a motorway, leaving three other passengers fighting for their lives.
Among the three people taken to hospital with life-threatening injuries were a five-year-old girl, a man and a woman, Thames Valley Police said.
A fourth person was admitted to hospital with less severe injuries after the collision on the southbound carriageway of the M1 near Newport Pagnell, between junctions 15 and 14.
Some of those on board the minibus were visiting the UK from India.
The drivers of the two lorries were arrested in connection with the incident in Buckinghamshire. A 31-year-old man from Worcestershire was arrested on suspicion of one count of causing death by dangerous driving and one count of drink-driving.
A passer-by who stopped his journey home to help assist a young girl trapped in the minibus said he had given his jacket to the child to keep her warm.
A 53-year-old from Stoke-on-Trent is also in custody, on suspicion of one count of causing death by dangerous driving.
It is thought the minibus had travelled from the Nottingham area, Thames Valley Police (TVP) said.
Chief Inspector Henry Parsons from TVP’s joint operations unit for roads policing, said: “The scene facing the officers who attended was complex and the recovery operation which followed was challenging. Tragically eight people lost their lives, with four further people seriously injured, and it is our duty to establish the circumstances of what happened.
“Officers are working to inform the next of kin of the people who died and specially-trained family liaison officers will be offering them their support. Some of those involved were visiting the UK from India and we are working to keep families abroad informed.
“This collision was deeply distressing for the members of the public who witnessed it, along with the emergency service workers who attended and displayed high levels of professionalism.”
Emergency services were called shortly before 3.15am on Saturday and the road was closed until the early afternoon to allow recovery work to take place.
Pictures of the vehicles appear to show a FedEx lorry with its cab crushed and the cab of an AIM Logistics lorry twisted around to the side. Another image shows what appears to be the crumpled wreckage of the minibus on the back of a flatbed truck. Broken glass and debris from the collision was scattered across the tarmac.
Ismail Elmagdoub, director of AIM Logistics, said: “One of our vehicles was involved in a serious incident on the M1 southbound, junction 14 this morning at approximately 3am.
“We would like to express our deepest sympathy to the families who have lost loved ones and also those whom have been injured at this very sad time. Road safety and compliance is of the utmost importance to AIM Logistics and we are continuing to assist the police as much as possible with their inquiries.”
A spokesman for FedEx said: “We are aware of the tragic accident that occurred last night on the M1 motorway, junction 14 in the UK with a FedEx truck involved. Our deepest thoughts and concerns are with all of those involved.
“Safety is of the utmost importance to FedEx, and we are cooperating fully with authorities but have no further information at this time.”
Anyone with information, to call the 24-hour Thames Valley Police inquiry centre on 101, quoting URN URN 203 of August 26.