A 16-year-old girl was fatally stabbed to the head and face before attempts were made to set her on fire in an alleyway.
A post-mortem examination showed college student Sasha Marsden, from Blackpool, Lancashire, died as a result of "significant stab wounds", police said.
The teenager was found in the South Shore area of the resort at about 9pm on Thursday.
Lancashire Constabulary said it has been given more time to question a 22-year-old man held on suspicion of her murder. A 20-year-old woman who was arrested at the same time on Thursday night has been released without charge.
A police spokesman said: "Following the death of Sasha Marsden, a Home Office post-mortem examination has been carried out and has found that Sasha died as a result of receiving significant stab wounds to the head and face.
"It also showed that attempts were made to set the body on fire and this was after Sasha had died. The results have been shared with the family, and specially trained officers are continuing to support them.
"Detectives have been granted a warrant of further detention in respect of a 22-year-old man and he will be detained at Blackpool police station for a further 36 hours. The 20-year-old woman who was arrested has been released without charge."
On Friday, specialist crime scene investigators carried out forensic searches of the alley behind Kirby Road and at the Grafton House bed-and-breakfast hotel on the same road, which is a few streets off the Promenade.
Sasha was in the first year of a childcare course at Blackpool and The Fylde College and lived with her parents in Staining, near Blackpool.
In a tribute to her, Mandy Pritchard, head of the college's school for society, health and childhood, said she was "a friendly, considerate individual who cared about her fellow students".
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