The full post-mortem examination on the body of Tia Sharp has failed to establish a formal cause of death.
Further tests will now need to be carried out, Scotland Yard has said.
A statement from the Metropolitan Police said: "This afternoon a full post-mortem at Croydon Mortuary has concluded.
"It did not establish a formal cause of death and we await the results of further tests."
Earlier, the body found in the loft of Tia's grandmother's house in New Addington, south London, was formally identified as that of the schoolgirl.
She had been missing for a week when forensic officers found her body wrapped in a bedsheet in a black bag.
Around 80 police officers had been combing the surrounding area and friends and neighbours joined the search.
The boyfriend of Tia's grandmother, 37-year-old Stuart Hazell, has appeared at the Old Bailey charged with murdering the 12-year-old.
Coroner Dr Roy Palmer, conducting the inquest into Tia's death, has said that Hazell now has the right to request a second post-mortem examination by a pathologist of his choice.
He added that if it was requested, it should be carried out as soon as possible so Tia's body can be released to her family.
The inquest, at South London Coroner's Court in Croydon, has been adjourned to a later date.
A provisional trial date for Hazell has been set for January 21 next year.
