The Metropolitan Police Commissioner says the failure to find schoolgirl Tia Sharp's body more quickly was not down to an individual officer.
Police teams searched the home of the 12-year-old's grandmother in New Addington, south London, four times before her body was finally found in the loft.
It is understood that the body was wrapped in bin bags when it was found.
The force has apologised to Tia's mother Natalie Sharp for the delay in finding her and put it down to "human error".
Bernard Hogan-Howe said: "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.
"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."
An internal review is being carried out and he said he hopes to have some answers within "a few weeks" about what happened.
"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."
Tia's grandmother's partner Stuart Hazell, 37, from New Addington, has been charged with her murder and is due to appear at the Old Bailey for a plea and case management hearing on November 19.