Police have warned that the case may never be solved
Police searching for Madeleine McCann have said they are pursuing a "significant line of inquiry" as the 10th anniversary of her disappearance approaches.
Three-year-old Madeleine disappeared from a holiday apartment in Praia da Luz, Portugal on 3 May, 2007.
London Metropolitan Police Assistant Commissioner Mark Rowley said investigators still hope they can provide answers to what happened.
"I know we have a significant line of inquiry which is worth pursuing,” he said. “Because it's worth pursuing it could provide an answer, but until we've gone through it I won't know whether we are going to get there or not."
"Ourselves and the Portuguese are doing a critical piece of work and we don't want to spoil it by putting titbits of information out publicly."
He warned however that the case may never be solved.
"Sadly investigations can never be 100 per cent successful," he said. "We will do everything we can do reasonably to find an answer as to what's happened to Madeleine. I so wish I could say that we'll definitely solve it. But a small number of cases sadly don't get solved."
Meanwhile, the only four official suspects investigated by the Met Police over the abduction have been ruled out of the inquiry.
The suspects were believed by Scotland Yard to have taken her during a burglary gone wrong at the McCanns' rented holiday apartment in Portugal.
They were identified by their mobile phone use, their location near the apartment on the night and their backgrounds.
One is Jose Carlos da Silva, 35, a former driver at the Ocean Club complex in Praia da Luz, where Madeleine vanished on 3 May, 2007.
He and the others were questioned and investigated for six months before being told they would face no further action.
Mr Rowley said: "Somebody's doing a burglary, panicked maybe by a waking child and that is what leads to Madeleine going missing."
Asked if that was an unlikely scenario and if any surprised burglar would simply run away, Mr Rowley said: "In my experience, if you try to apply the cold, rational logic of a normal person sitting in their front room to what criminals do under pressure, you tend to make mistakes.
"It was a sensible hypothesis and it's not entirely ruled out."
Madeleine disappeared as she slept in a room with her younger twin siblings while their parents Kate and Gerry McCann, both doctors, dined with friends nearby.
Scotland Yard began investigating six years ago after the failure of the first Portuguese police investigation.
Mr Rowley said new information was still being received daily.
Portugal's deputy national director of the Policia Judiciaria, Pedro do Carmo, told Sky News that he was under no financial or political pressure to wind up his re-opened investigation.
He said: "If the Metropolitan Police decides to close its investigation that doesn't mean we are going to close ours.
"Our two investigations are not dependent on one another."
Kate and Gerry McCann said the 10-year anniversary was a "a horrible marker of time, stolen time."