Human remains were found in two suitcases when police responded to a call about a driver acting suspiciously, officers have said.
Gloucestershire Police was called to near Coleford in the Forest of Dean just after 22:30 BST on Tuesday.
The vehicle was identified, a man and a woman were questioned and then arrested, the force said.
Officers believe the remains are that of a woman but forensic examinations are ongoing to identify the victim.
Earlier, detectives were given an additional 36 hours to continue questioning the man, who is in his 30s and from Wolverhampton, and the woman, who is in her 20s and from Birmingham,
Det Ch Insp John Turner said: “The nature of this incident is distressing and we’re working around the clock to fully understand what has happened.
“Someone’s life has been lost and our priority is to identify the victim and get answers for her family.
“Searches have taken place in the surrounding area for evidence gathering and contrary to media reports no remains have been found as part of these searches.”
Road closures on the A4136 are expected to remain in place until Friday, officers said.