Five police officers have been told their behaviour may have breached standards after a student died in custody.
Nuno Cardoso, 25, suffered a "medical episode" in a police vehicle after he was arrested on suspicion of assault on 24 November.
The law student, who studied at Ruskin College in Oxford, died in hospital.
The Independent Office for Police Conduct said its investigation was "nearing its conclusion".
"We are awaiting the final post mortem report and expert evidence. All other lines of inquiry are complete," a spokeswoman added.
"Five police officers have been informed their conduct may have breached the standards of professional behaviour.
"This does not necessarily mean that misconduct proceedings will follow."
The watchdog previously said it was looking into whether the decision to transport Mr Cardoso to custody rather than seek medical assistance at that point was appropriate.
Ruskin Students Stand Up to Racism has called for a "transparent and open" investigation into the death.
College principal Paul Di Felice said Mr Cardoso was a "valued student" and "highly regarded by his fellow law students".
A crowdfunding page has been set up to raise money to help his family.
It describes the student as "a high spirited, caring, and good natured young man" who wished to become a lawyer.
Thames Valley Police said it could not comment while the investigation is ongoing.