Madeline McCann is believed to be dead following her disappearance 13 years ago.
German police are treating the case as a murder investigation and have named sex offender Christian B as their prime suspect.
But why were Madeleine McCann's parents suspects back in 2007?
Here's everything you need to know...
Why were Madeleine McCann's parents suspects?
Portuguese cops made Kate and Gerry McCann formal suspects (arguidos) four months after her disappearance.
Officers claimed to have DNA evidence as grounds for suspicion.
The decision came after police sniffer dogs, trained to detect human blood, barked inside the family's holiday apartment.
According to officers, the animals were taken to several locations but only reacted inside the McCann's rental home.
Later, the cadaver dogs reacted again while inside Kate and Gerry's hire car – which they rented 24 days after Madeline's disappearance.
The UK's Forensic Science Service (FSS) tested hair and fibre samples found in the apartment and car.
After sending off the samples for testing, Madeline's parents were named as suspects.
What was the Madeline McCann's parents theory?
Portuguese police reportedly suspected that Kate and Gerry accidentally killed their daughter.
It was suggested that they had given the three-year-old sedative so that she would sleep while they dined with friends some 50 yards away.
When they returned back to their holiday rental, it was alleged that they hid the body and faked Madeline's abduction.
They were accused of hiding her body in the boot of their rented car and buried her weeks after the investigation started.
Cops allegedly believed that this explained the sniffer dogs' reaction inside the vehicle.
Crime expert Moita Flores, who previously worked on the Portuguese investigation, claimed that Madeleine died in the apartment.
He told Portuguese website Flash!: "Maddie died in that apartment, I have no doubt.”
"It would have been impossible to get through a window with a child."
Flores' comments echoed claims previously made by retired Portuguese police chief Goncalo Amaral.
In 2008, Amaral published a book called Maddie: The Truth of the Lie.
The novel accuses Kate. and Gerry of faking Madeline's abduction after accidentally killing her.
Are Madeleine McCann's parents still suspects?
Portuguese police remove Kate and Gerry's arguido status in July 2008.
The couple have always maintained their innocence.
After examining the DNA evidence submitted by cops, John Lowe of the FSS explained the samples were not entirely reliable.
He said: "The individual components in Madeleine’s profile are not unique to her; it is the specific combination of 19 components that makes her profile unique above all others.
:Elements of Madeleine’s profile are also present within the profiles of many of the scientists here in Birmingham, myself included.
"It’s important to stress that 50 per cent of Madeleine’s profile will be shared with each parent.
"So it is not possible, in a mixture of more than two people, to determine or evaluate which specific DNA components pair with each other.
"Therefore, we cannot answer the question: Is the match genuine, or is it a chance match."
Reliability of sniffer dogs has long been the subject of debate.
During one US court case, a judge agreed that the animals were wrong 78%, 71% and 62% during one analysis.
Madeleine McCann: The Hunt for the Prime Suspect is on ITV tonight (July 23) at 9pm
If you miss the documentary, you can catch up on ITV Hub afterwards.
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