A post-mortem examination will be held on Friday to determine the cause of death of a young woman who died in Derry in the early hours of Thursday.
The 19-year-old died after being taken to Altnagelvin Hospital.
Detectives are appealing for information to trace her last movements.
Detective Chief Inspector Michael Harvey, who is leading the enquiry, said: “The death of any young person is always tragic, and I would first of all like to express my condolences to the relatives and friends of the young woman.
“She was aged just 19 and was from the Co Down area. She was in Derry as part of her university studies to become a nurse.
“What we know so far is that shortly before 2.30am, paramedics were called to a flat at Cromore Gardens.
“They found the young woman unconscious at the foot of some stairs. She was taken to hospital, where she sadly died.
“We are trying to establish her movements before 2am. We know that she was in a nightclub in the city centre with friends. We believe she left the nightclub in a taxi at about 1am.
“We are keeping and open mind as to the circumstances of this tragic death.
“If anyone saw anything unusual in the Cromore Gardens area, we would like them to contact the detectives at Strand Road police station by calling the 101 non-emergency number and asking for extension 57072.”
Police have not formally released the name of the young woman.
The 21-year-old man who was arrested earlier today remains in custody at the Antrim serious crime suite.
Local Sinn Fein councillor who was one of the first on the scene was in direct contact with the young girl’s family.
“I have spoken to the girl’s uncle and as you would expect the family are devastated.
“The girl was only 19, she was from Portaferry and was in Derry studying to be a nurse at Magee, she had her whole life in front of her - this is so tragic,” said Colr. Campbell.