Evolve or Die! 4.

Scruff and I recovered from our experience quite quickly, and I got Scruff doing what she does best - hacking into police files to find out if this was truly the isolated incident we had been assured it was. After all, the police didn't want to cause panic by revealing that unexplained deaths were taking place in London.
Her first stop was MI5, the Security Service, using a very illegal way in. If the girl's death was linked to any known group, MI5 would know.
Scruff spent ten minutes searching MI5. I watched her face. It looked bad.
"Is it Al-quaeda? I asked. She shook her head.
"No, it's worse. MI5 are baffled, like us."
"Ty the Police."
"They know nothing about it either. Sis, this is really, really bad."
"I know. Were there any other smilar deaths?"
"Three, all in London, all drug addicts."
So it was targtted!
"A bad batch of heroin?"
"No, that's the really weird thing. One was addicted to crack cocaine, another to cannabis, and the third to heroin. Those drugs come from different parts of the world, by different routes. Cocaine comes from South Ameica, most heroin comes from Aghanistan, and most cannabis is grown in this country. They're all treated differently too, processed differently. It's hard to believe they could all three be contaminated by mistake."
"It's more than that," I told her. "It's impossible."
"And I remember the old Sherlock Holmes rule - once you eliminate the impossible, whatever remains, however improbable, must be the tuth. Sis, you're saying this is deliberate?"
"It has to be."
"Why?"
"The four victims were connected. They have to be."
" They were all drug addicts, that's..." Scruff tailed off. "Sis!"
"Exactly. We're facing a deadly serial killer who targets drug addicts. We have to stop them - before hundreds die."
Yet I was not even sure where to begin.