An employee at the resort where Madeleine McCann disappeared could have information that will solve the case, according to a Scotland Yard police officer.
Officer Colin Sutton told Channel 7's Sunday Night programme that a former worker could give crucial insight into the case.
"There is an employee, somebody who worked within the Ocean Villa complex, who has some information or some knowledge that may be of assistance," he said.
Police are searching for the employee, who has not been named, and it is unclear if they are male or female.
Madeleine disappeared from her bed at Ocean Club resort while her family were holidaying in Portugal.
Her parents were eating in a nearby restaurant at the time.
Mr Sutton put forward five theories about what happened to the girl, including that she could have been taken by a paedophile or taken by a trafficking gang.
He said it would be extremely difficult to find her body if she had died in Portugal.
"It's a large area of very low population, of scrubland, of ancient wells.
"There are areas there where, you know, humans probably don't go, from one year - or one decade, even - to the next."
McCann's parents Kate and Gerry told Sunday Night they had nothing to do with the girl's disappearance, and continued to vehemently deny that they are suspects.
Mr Sutton said he has come up with five plausible theories to explain Madeleine's disappearance.
The detective claimed she could have been taken by a paedophile or she could have been snatched by a trafficking gang to replace a dead child.
Two weeks ago a former nanny of Madeleine McCann's spoke to The Mirror about the night the three-year-old disappeared from her Portugal hotel room.
The nanny defended Madeleine's parents, who had left the girl sleeping in a room with her siblings while they dined at a nearby restaurant.
She said it was very normal for parents to do this at the Ocean Club Resort, and said "there's no way at all" and "not a chance" that McCann's parents are in any way responsible for whatever happened to her.
The nanny also said she felt that the resort was unsafe and she was told by fellow staff to never venture off the site alone, and had been given a rape whistle to carry with her at night.
In October 2015, the case investigating Madeleine's disappearance was scaled down from 29 to four detectives.
To date £12 million (NZ$21m) has been spent on the investigation, and Madeleine's parents say they will never give up hope of finding her.