Metropolitan Police Commissioner Sir Bernard Hogan-Howe has defended the way Portuguese police handled the initial investigation into the disappearance of Madeleine McCann.
The country's most senior officer said it would have been "very difficult" for officers in Praia da Luz to know whether they were dealing with a serious crime immediately after the little girl vanished.
Speaking on LBC 97.3 this morning, he said: "I think sometimes these things at the beginning can be very difficult to deal with, you don't know exactly if the child has just wandered off. It can be very difficult to know if you've got a very serious crime.
" I'm sure for them that must have been a challenge. Anybody can go back after two, three, five, six years and say 'why didn't you do that'?
"That's easy in hindsight.
"We don't like it when it happens to the Met, and I'm certainly not going to do it to the Portuguese. What I'm determined to do is to work together to make sure we've got the best chance to now try to get to the bottom of this terrible tragedy.
"There is a poor family there who've got the torture of not knowing whether their daughter is alive or not."
H e said the discovery of two blonde-haired, blue-eyed girls living with Roma families in Ireland and Greece has not had any "direct impact" on the investigation, but added: "It looks like there's a possibility that around the world there are occasions when people steal children and keep them.
"And I'm sure in one sense that's what the McCanns must be hoping, that at least their child is alive.
"The other possibility is that sadly she's not alive, and either way we want to try and end the torture that they're going through."
Scotland Yard is now running its own investigation into what happened to Madeleine, who went missing from a holiday apartment on May 3 2007 as her parents dined at a nearby tapas restaurant with friends. The Portuguese inquiry was shelved in 2008.
Refreshed appeals were made last week as part of the British investigation, with Detective Chief Inspector Andy Redwood appearing on television in the UK, Holland and Germany.
He revealed that his team had discounted a previous sighting by the McCanns' friend Jane Tanner, which was thought to be an abductor carrying Madeleine away and had been put at the centre of the Portuguese investigation.
His team managed to track down the man - an innocent British holidaymaker - and instead decided to focus on a second suspect who was also seen with a child in his arms.
Officers also said they would like to trace a number of fair-haired men who were seen lurking near the apartment at the time, and are looking at a surge in burglaries in the area as well as bogus charity collectors.
An appeal is due to air on Irish television later this month.