A delay in finding schoolgirl Tia Sharp's body was not down to an individual officer, the Metropolitan Police Commissioner has said.
Police searched the home of the 12-year-old's grandmother in New Addington, south London, four times before her body was finally discovered in the loft.
She had been missing for a week when forensic officers found her wrapped in a bedsheet in a black bag.
The force has again apologised to Tia's mother Natalie Sharp for the delay in finding her and put it down to "human error".
Police chiefs have launched a review into why it took so long to locate her, which they say will take "a few weeks" to reach any findings.
"If we thought it was an individual human error then that would explain it, but we're carrying out a review because we're not happy that explains it," said Commissioner Bernard Hogan-Howe.
"We're trying not to have a knee-jerk reaction, we need some time to understand what happened and what failed to happen.
"We want to learn lessons from this particular case. We have apologised and I repeat that apology today to Tia's mum and family for not having found Tia's body sooner."
He added: "We've explained that it was human error but we want to go into it more.
"You can always blame the individual but we want to understand what processes and management decisions we've made that led to that failure.
"That's why we're carrying out a review and we hope in a few weeks' time we will have some conclusions from that, so that we make sure it doesn't happen again."
Tia's grandmother's partner Stuart Hazell, 37, from New Addington, has been charged with her murder. He is due to stand trial on January 21 next year.
The full post-mortem examination on Tia's body has failed to establish a formal cause of death.
Her inquest, at South London Coroner's Court in Croydon, has been adjourned to a later date.