A German child sex offender may have kidnapped British girl Madeleine McCann after breaking into her family's holiday apartment in Portugal, police have said.
The male suspect, who has not been named, is described as white with short blond hair, possibly fair, and about 1.8 metres (6ft) tall with a slim build at the time she vanished on 3 May 2007.
Christian Hoppe, from Germany's Federal Criminal Police Office (BKA), told the country's ZDF television channel the 43-year-old is serving a prison sentence for a sex crime and has two previous convictions for "sexual contact with girls".
Mr Hoppe said German police have not ruled out a sexual motive, but added that the suspect may have broken into an apartment in the Ocean Club complex in Praia da Luz - where Madeleine was on holiday with her parents, Kate and Gerry McCann, and her twin siblings Sean and Amelie - before spontaneously kidnapping her.
The German national is known to have been in and around the area on the Algarve coast at the time Madeleine went missing shortly before her fourth birthday.
A half-hour phone call was made to his Portuguese mobile phone around an hour before Madeleine is believed to have gone missing.
The suspect has been linked to an early 1980s VW T3 Westfalia camper van - with a white upper body and yellow skirting, registered in Portugal - which was pictured in the Algarve in 2007.
British police said he was driving the vehicle in the Praia da Luz area in the days before Madeleine's disappearance and is believed to have been living in it for days or weeks before and after 3 May.
He has also been linked to a 1993 Jaguar XJR6 with a German number plate seen in Praia da Luz and surrounding areas in 2006 and 2007.
The day after Madeleine went missing, the suspect had the car re-registered in Germany under someone else's name, although it is believed the vehicle was still in Portugal.
Both vehicles have been seized by German police.
London's Metropolitan Police has launched a joint appeal with the BKA and the Portuguese Policia Judiciaria (PJ), including a £20,000 (€22,000) reward for information leading to the conviction of the person responsible of Madeleine's disappearance.
The investigation has identified more than 600 people as potentially significant and were tipped off about the German national, already known to detectives, following a 2017 appeal 10 years after she went missing.
She vanished shortly before her fourth birthday and would have turned 17 last month.
German police are treating her disappearance as a murder investigation but the British police investigation, launched in 2013, has always considered the case a missing person inquiry.
A statement from Madeleine's parents, read by Detective Chief Inspector Mark Cranwell, said: "We welcome the appeal today regarding the disappearance of our daughter Madeleine.
"We would like to thank the police forces involved for their continued efforts in the search for Madeleine.
"All we have ever wanted is to find her, uncover the truth and bring those responsible to justice.
"We will never give up hope of finding Madeleine alive, but whatever the outcome may be, we need to know as we need to find peace.
"We will be making no further comment in relation to the appeal today.
"We would like to thank the general public for their ongoing support and encourage anyone who has information directly related to the appeal to contact police."
Mr Cranwell told reporters that he was taking the "really unusual" step of releasing two mobile phone numbers as part of the appeal.
The first is believed to have been used by the suspect and received a call from another Portuguese mobile while in the Praia da Luz area, starting at 7.32pm and ending at 8.02pm on the night of 3 May 2007.
Madeleine is believed to have disappeared between 9.10pm and 10pm that evening.
The caller, who is not thought to have been in the Praia da Luz area, is not being treated as a suspect, but is said to be a "key witness".
"Any information in relation to these mobile numbers during the spring and summer of 2007 could be critical to this investigation," said Mr Cranwell.
"Some people will know the man we are describing today, the suspect in our investigation. I'm appealing to you directly.
"You may know, you may be aware of some of the things he has done. He may have confided in you about the disappearance of Madeleine.
"More than 13 years have passed and your loyalties may have changed.
"This individual is in prison and we are conscious that some people may have been concerned about contacting police in the past. Now is the time to come forward.
"I'm appealing to you to contact us, or the German authorities or the Portuguese authorities.
"I should be very, very clear on this - while this male is a suspect, we retain an open mind as to his involvement."
The suspect is known to have been linked to the Praia da Luz area between 1995 and 2007, with some short spells in Germany, and is described as having a "transient lifestyle", living in his camper van for days at a time.
An appeal on German Crimewatch-style programme XY said he is thought to have worked odd jobs, including as a waiter, but also committed burglaries in hotels and holiday resorts and dealt drugs.
He was also linked to two houses in Portugal - one between Praia da Luz and Lagos and a second inland, according to the appeal.
Police are appealing for anyone who may have seen the camper van in or around Praia da Luz on the night
Madeleine went missing, or in the days before or weeks after.
Detectives also want to speak with anyone who saw the van together with the Jaguar, or individually, during the spring and summer of 2007.
Deputy Assistant Commissioner Stuart Cundy said the investigation into the suspect had seen Met detectives sent to Portugal and Germany.
He added: "This is a significant development, which is why we've made the decision to take this significant step of making a public appeal on some information that we would not normally talk about in our major investigations."