Here's some information I hadn't seen before, which adds weight to the theory Madeleine's abduction was not a one-man crime. The original Bild article about AB is behind a paywall so I can't check the accuracy.
Maddie case: Investigators follow new trail - Christian B. is not extradited
*snipped*
Missing Maddie: Investigators had been tracking Christian B. for years
Update of June 5, 6:10 p.m .: Christian B., the suspect in the Maddie McCann case , has been under investigation for several years. The police are said to have searched the apartment of a former roommate, Alexander B., in 2018.
Two years ago, Alexander B., with whom Christian B. lived after Maddie's disappearance, was first questioned about a capital crime by the suspect
The flatmate said he knew nothing of Christian B.'s crimes , reports
Bild . He only told him about a diesel theft in Portugal. The suspect had boasted of his actions before (update from 3:00 p.m.).
A year later, the investigators then spoke to the acquaintance directly about Maddie .
He was informed that he would be investigated if Christian B. was kidnapped and murdered.
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I also (wrongly) believed CB & AB only became acquainted when the Jag was re-registered but it seems they had known each other for several months. (Sorry if the following article has been posted here previously way back in June)
Fall Madeleine McCann: Das sagt der Mitbewohner von Christian B. über den Maddie-Verdächtigen