Aktenzeichen XY ... ungelöst (Case number XY ... unsolved) airs once a month in Germany, on Wednesday evenings, and has been on air uninterrupted since 1967. The show on public broadcaster ZDF is credited as the inspiration for shows like "Crimewatch" in the UK or "America's Most Wanted" or "Unsolved Mysteries" in the US. 

The focus is on a lengthy and detailed reconstruction of some of the toughest crimes, made in collaboration with law enforcement, incorporating appeals for further relevant information from the public. 

Wednesday's edition of the 90-minute program was broadcast at the same time as German investigative police broke the news that they had identified a murder suspect in the long-running McCann case, currently identified only as a 43-year-old German with a prior sex crimes record facing a separate prison sentence.

Read more:  German sex offender identified as suspect in Madeleine McCann disappearance

McCann's case, as well as her parents Kate and Gerry McCann, had already appeared on the show in October of 2013 — with that episode drawing almost a quarter of the total German TV audience that night.

Police even acknowledged that the current suspect was first identified as a result of people calling in after that show. However, there was not enough evidence to pursue him in earnest at the time, with more information coming to light by 2017. 

Read more:  Madeleine McCann case: the key events since her 2007 disappearance

Almost 5,000 cases featured, around 40% solved 

According to data published by ZDF at the end of 2019, the show had tackled 4,752 different cases across 549 episodes — with 1,905 of them having since been solved. ZDF called this success rate in the region of 40% "a proud figure because often it's the 'hopeless' cases which wind up on XY — cases, in which police have already exhausted all the traditional methods of investigation and can only really build on the help provided by the television audience."

Almost one in three cases the show tackles are murder or homicide. German investigators are currently working on the assumption that Madeleine McCann — still formally a missing person — could prove another. Robberies are the next most common category investigated, followed by fraud-based crimes, attempted homicides, sexual crimes, and missing persons cases. In every category, the show claims a success rate north of 30%.

International connections are also by no means uncommon. For decades, via a broadcast partnership, the show was regularly aired on Austrian and Swiss public television for their German-speaking audiences, although these agreements ceased in 2003.

Foreign police often turned to the show for help, particularly in cases with a suspected or potential German angle — as with the McCann's case given Germans' propensity to vacation in and move to the Algarve area of Portugal. Britain had turned to the show in the hope of finding information on seven occasions, including McCann, with a total of 16 other countries bringing unsolved mysteries to the show.

To provide a further flavor, here's a brief selection of some of the show's more renowned past cases.

Sigrid Paulus — missing person?

The 40-year-old disappeared from her home in Königswinter, near Bonn, in 2008. Her husband, Gerd, did not report her missing, saying instead that she had left the family in search of a new life, citing arguments over money problems. By 2012, Paulus' daughter turns to the media, and a journalist then gets the police involved. Aktenzeichen XY investigates and later goes to air confirming that Sigrid never registered at a new place of residence (a legal obligation in Germany), never used her health insurance, and generally seems to have dropped off the map. 

After the broadcast, Paulus' neighbors call in to say that her husband did substantial digging in his garden right after the disappearance. The prosecutor's office approves investigative excavations at the family home. After opening up the cellar walls and floor, investigators find Sigrid Paulus' corpse. Gerd Paulus confesses and is sentenced in 2014 to eight years in prison on charges akin to manslaughter or second-degree murder.

Lolita Brieger — decades-old mystery solved, but justice not served

Born in 1964, and vanished in 1982. She'd met a new beau, the son of a wealthy farmer, in 1981, aged 17. His family didn't approve of the match.

By 1982, they had separated, but Lolita was pregnant with his child. She attempts suicide on learning about this and disappears not long after. Her family reported her missing and the ex-boyfriend is a suspect. The trail went cold initially, but the case stayed on the book as a potential murder trial. The Trier police commissioner at the time also promises Lolita's mother not to give up on the case. By 2011, after fresh leads in this coldest of cases, the crime is featured on Aktenzeichen XY. 

One of the subsequent callers says she knows a good friend of the boyfriend, who might know more. The friend is questioned and admits having helped the ex-boyfriend dispose of the body: He takes the police to a rubbish tip, where they find the skeleton wrapped in plastic. DNA confirms it is Lolita, and the ex-boyfriend is arrested.

He went on trial in 2012 but was let go, because of insufficient evidence of first-degree murder. As in many countries, only "Mord" (comparable to first-degree murder in the US) stays on German books indefinitely. The statute of limitations for lesser charges had expired. 

The mafia murder unraveled by a ship's propeller

Not all of the cases that are solved necessarily end up in conviction. In 2012, the show helped disprove German police theories about potential organized crime murder. 

Police had fished a headless torso out of the Rhine river, near Duisburg, in 2011. They found the limbs and head to have been cleanly severed and therefore presumed an organized crime murder connection. Their investigations were further hampered by a miscalculation, meaning that they were initially seeking a far taller man than the deceased individual. 

Aktenzeichen XY focused in particular on this measuring error in their feature, appealing for fresh leads. A viewer called in, saying that his neighbor had been missing for the exact same period of time. Police went into the man's apartment, found DNA and confirmed the match to a short, 74-year-old pensioner. He had either accidentally fallen into the river, or committed suicide — a passing ship's propeller sliced up his body, giving police the mistaken impression of foul play.