One guy alone couldn't have done this

You've gone through all the facts and come to the conclusion, that there must be more people involved in this crime. You could be right, though. The later victim was surrounded by people in the pub. And as she was kind of new in that town, she had not many friends yet. There was Isabella, a girlfriend she knew from nursing school. Isabella was with her boyfriend and his colleagues. 
These people have been looked at very closely by the police. Because some of them worked as soldiers at the British Army base, the investigation was handed over to the SIB, the Special Investigation Branch of the British Army. They didn't come up with anything. Don't get me wrong. These guys were pro's at the time, they would have found something, if there had been anything. But the pub was crowded that night and the guys who eyeballed her could be total strangers. It can be caused by group dynamics. One guy brings the liquid ecstasy and says to his buddy: "But you have to put it in her glass." It is often that victims after being poisoned try to get out of the location, to breath fresh air. Our two or three buddies only have to walk after her and catch her up before she loses consciousness.

And then they were three (or four)

Minutes after Frauke leaves the pub, two or three guys follow her discreetly. They walk a few meters behind her. After a short while Frauke feels dizzy. Maybe she tries to sit down for a while. And here they are. Our three heroes of the night. They ask her what's wrong. And then they offer her to bring her home. Come on. Let's take the car. In the car Frauke passes out completely. They drive out of town to Nieheim. They abuse her, one after another. After that they go through her belongings. Especially one thing interests them. Her cellphone, which she had been using in the pub constantly.
The cellphone battery is empty so they have to charge it. Back then it wouldn't be a problem, because almost everyone had a charging cable in his car. 
They assemble a SMS out of sent stuff or drafts, they found on the phone.
"Coming later. The game was funny not against England :) LYVM see you later"
They send this to her closest contact.
Why is this important:
If they are 40 kilometres away, they wouldn't have to worry about someone or the police looking for them.
They have to worry and write such a text message, because they aim to go back to town where they live.
Or something totally different happened. What do you think?

Who takes her

The two or three guys discuss whether they should let her go after they had their joy with her. One or two of them are against it, afraid that she could recognize them later. They decide not to let her go.
But where should they hide her. One of them has a place. A garden arbor or a summer house or a garage. They bring her there, tie her up, duct tape her. Maybe one of them stays with her all night. Frauke is passed out and won't wake up until early afternoon the next day. 
After she wakes, up one perpetrator brings her water and maybe food. They don't talk much. Frauke doesn't know the guy and he doesn't know her. He would have even let her go, but he can't. The others would kill him. But he thinks about that.
And after a while it is five p.m.. The other guys finish work and come over to the place.
And the whole ordeal starts all over again. It's what they call "having fun". Only this time Frauke is less sedated. She catches up some of their conversations, what to do next, and tries to use it to her advantage. "If they hear from me they won't look for you", she argues for her life. She is successful. They wait until it's dark put her in a car and bring her to a remote spot to make the  phone call.

The phone calls

Frauke: "Hello Christos, I wanted to say that I'm fine and that I'll be home soon. Let mom and dad and the others know." The unusual name Christos instead of Chris could be a hidden sign. "OS" is a licence plate code for a county that lies 70 kilometres northwest of Paderborn.
There are these strange repetitions in the calls and text messages. Here's a list of terms and phrases she uses in a repeating way:
to / at home: eight (times)
No: seven
Paderborn: five
yes: five
later/late: five
Mom: five
can't say: four
can't: four
explain: three
don't ask: three
don't know: two
Dad: two
If this is a code, you could decrypt it like this:
"I don't know, Dad.
Don't ask for explanations.
Can't say, can't.
Mom later yes.
Paderborn 
No
At home"

They can't get enough of her 

It's hard to say what motivates them to repeat the same actions in the following days. Text messages and calls at night from different places bringing the hostage with them. For the perpetrators it must be very stressful risking every night to be seen by witnesses or pulled over by the police. This behavior could be caused by Frauke manipulating the perpetrators. Or it originates inside the group, with one faction wants her dead and the other doesn't. However, she stays alive and sends signs of life. A life that consists only of abuse and torture.
The group has a captain, a first officer and probably one or two useful idiots. The motive is sexual, but goes beyond that. They had their fun with her two times, three times. What drives them to go further? 
A) They never killed anyone and are afraid to do so.
B) It satisfies their sadistic desires to keep her alive.
Maybe it's a mixture of both or it's something completely different.

How it ends

Let's skip the next five days of misery and take a brief look at how this martyr ends. As you know there was no blunt force trauma found on her body, no stabbing or gun shot wounds either. How did she die then? Did she die of thirst? Or of an overdose of liquid ecstasy? Was she drowned or had a plastic bag put over her head and suffocated?  The forensics didn't even found signs for strangulation. This behavior shows the emotional distance the group had towards its victim. What caused this distance? Were there communication issues, like they didn't speak her language and she did not speak theirs. Or were the perpetrators mentally disabled? Or there was a kind of discipline in the group that might be based on a military background they were all part of. We mentioned the hierarchy before. Ok. Now it's time to make the first profile of the group leader.

Made with Mobirise site creator