Eve Smith, 21, Rafel Jeanne, 24, and Darcy Ross, 21, died after the accident.
Image source, Social mediaDetectives have confirmed the victims of a crash that killed three were trapped for almost two days.
Police said CCTV was studied and number plate recognition cameras used to establish the crash happened at 02:03 GMT on Saturday, 4 March.
It was not until 46 hours later the victims were found, just after midnight on Monday morning.
Eve Smith, 21, Rafel Jeanne, 24, and Darcy Ross, 21, died after the accident near the A48 in St Mellons, Cardiff.
Image source, Social Media
Sophie Russon, 20, and Shane Loughlin, 32, remain in a critical condition.
A sixth person, who had been with the five who were in the crash, had been dropped at home earlier.
The three women, from Newport, had gone to The Muffler club in the city’s Maesglas area late on Friday.
They then travelled 36 miles (58km) to Trecco Bay Caravan Park, in Porthcawl, Bridgend county, with the two men, both from Cardiff.
A first missing person report was made to Gwent Police at 19:34 GMT on Saturday, with further missing person reports made to the same force at 19:43 GMT and 21:32 GMT on Saturday.
A further missing person report was made to South Wales Police at 17:37 GMT on Sunday.
The IOPC said on Tuesday it was investigating the actions of South Wales Police and Gwent Police.
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Ms Russon’s mother Anna Certowicz has said police “didn’t seem to think it was worth investigating” when the five people were reported missing.
Eve Smith’s dad, Everton Smith, said his life had been changed forever by his daughter’s death.
The accident happened after the white Volkswagen Tiguan the five were in left the carriageway and entered a wooded area.
Post-mortem examinations are ongoing.
Assistant Chief Constable Jason Davies said: “The investigation is making good progress in piecing together the events leading up to the collision.
“Specialist officers will continue to carry out a detailed investigation which will enable us to provide the facts of what happened during the early hours of Saturday morning.”
Police are appealing for witnesses and dashcam footage.