"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.
"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 and is due to appear at the Old Bailey for a plea and case management hearing on November 19.