Madeleine McCann's parents have called on police to tell them about evidence that reportedly contradicts the belief that their daughter was abducted.
Madeleine, four, has been missing since 3 May
In Portugal, press reports - attributed to police sources - say detectives believe she is dead. Publicly, police are refusing to comment on the reports.
The four-year-old's father, Gerry, told BBC News he was unaware of any "evidence of serious harm".
On Saturday, the McCanns will mark the 100th day since Madeleine disappeared.
Meanwhile, suspected traces of blood from the Algarve apartment where Madeleine was last seen are being tested in Birmingham by UK scientists.
Under Portuguese law, police are not allowed to speak publicly about the details of an investigation.
But in recent days, several newspapers have printed articles in which unnamed police sources have suggested Madeleine might have died in the family's holiday flat in Praia da Luz on 3 May - the night she disappeared.
In a BBC interview, Mr McCann acknowledged that there appeared to be a shift in detectives' thinking running contrary to what he and his wife had always been told.
"I had a fairly strong belief that there was a lack of evidence, certainly initially, that there was no evidence of serious harm and I do not know of any specific evidence now that alters that.
"I think, as parents, if there is evidence then we need to know about it."
He also told Portuguese television that the couple would not accept Madeleine was dead "until there's concrete evidence to that effect".
The couple have met detectives twice this week but are understood to be frustrated by a lack of clear information.
Police will not publicly comment on reports that Madeleine is dead.
But a spokesman has said there are indications to contradict the view that she was abducted.
Mr McCann told the BBC: "Clearly there is a shift in the investigation, which has led to the recent activity.
The McCanns say they still believe their daughter is alive
"One of the good things has been the collaboration between the Portuguese and the British.
"This is the first time a foreign police force has been allowed into Portugal to work alongside the Portuguese police."
Mr McCann said it was also "incredibly difficult" to deal with speculation that Madeleine may be dead and that the couple may have been involved.
"That is just unbelievable to try and cope with," he said.
Madeleine's mother Kate said: "All we want is Madeleine back, and the truth to be out. Anything else we'll stay strong and we'll get through."
Meanwhile, on Friday the McCanns will help to unveil a new section of the video-sharing website YouTube for footage of missing children.
The Don't You Forget About Me channel will be managed by the International Centre for Missing and Exploited Children in Washington DC.
The new channel includes an updated version of the Find Madeleine campaign video, featuring images of the child to the soundtrack of the Simple Minds song Don't You Forget About Me.
There are a number of other videos on the site appealing for information about children missing around the world.
The main YouTube site already hosts more than 200 videos related to the Madeleine campaign.
One of the videos has been viewed more than 50,000 times.