Rose Farley died in the wake of police warnings over potentially lethal 'pink ecstasy' pills.
The teenager from Liverpool, fell ill at a birthday party on Friday evening and died at Alder Hey Children's Hospital during the early hours of Saturday morning.
A 16-year-old youth from Liverpool was detained and questioned on Sunday in connection with the death.
But Merseyside Police said he and a 17-year old from Everton, who was arrested on suspicion of manslaughter and of supplying controlled drugs, had been released on unconditional bail.
It is understood Rose attended a party at the Silvestrian Social Club and that her drinks may have been spiked.
A Merseyside Police spokeswoman said the death of the youngster was still being treated as "unexplained".
She added that although a post-mortem examination had been carried out, the cause of Miss Farley's death has been withheld pending toxicology results.
A worker at the Silvestrian Social Club, who did not want to be named, told Sky News that staff were not aware that anyone had been taken ill on the premises.
"It was a busy night, an 18th birthday party, and as far as anyone knew the evening passed off without incident," she said.
"We certainly didn't know about anyone being taken ill. Everyone was gone by 1.30am."
Detective Superintendent Tim Keelan of Merseyside Police said: "At this stage it is too early to draw conclusions about the cause of death and it is unhelpful to speculate.
"The implication that this may be drug-related is just one of the lines of inquiry that our comprehensive investigation is looking at.
"That said, I would like to take this opportunity to warn people about the dangers of taking illegal substances.
"The effects of drugs are not always immediately apparent or can be delayed.
"This can sometimes result in people taking more in order to speed up the effects. This can have devastating results, one of which can be death."
The force had warned of a potentially lethal new drug on the streets in the form of pink pills, known as pink ecstasy, Dr Death or Pink McDonalds - because the pills are embossed with an M.
The warning was issued following the sudden deaths of two men in 24 hours in Cambridgeshire earlier this week. A man also died in similar circumstances in Bournemouth, Dorset, on May 21.
Emergency workers were called to Miss Farley's home during the early hours of Saturday morning. She was rushed to hospital but died just after 5am.
More than 4,500 Facebook users have viewed a tribute page set up in her memory.