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"So how did you know it was Culverton and Smith?"

That was the first question John asked after both she and Sherlock composed themselves enough to stop crying and Sherlock stopped pawing at John to assure himself that John—and Benedict/Beatrix—was alright.

"Lestrade frequently complained about Smith's egregious wizardness and his utter lack of effort to stifle it," said Sherlock. "Even reporting him to his superiors and providing a uniform for him to wear didn't stop him. Why such a stubborn refusal to change? There were three possible explanations: Smith is unprofessional, prejudiced, or he had a more sinister reason to keep up his fashion faux pas. I might have considered the first two options had it not been for the fact his partner Culverton had no such troubles. Culverton should've at least voiced token protest if only to make his own self look good. But he didn't. If anything, he aid and abetted Smith by failing to report his behavior to the Magical Law Enforcement. The two were in it together, then. I realised the scheme the moment Lestrade told me Smith had been erasing DI Morton's memory on a daily basis: Smith kept his seeming lack of expertise in Muggle dress to give himself an excuse to Obliviate the Muggle lead investigator and stall his progress."

"And it would've worked if it weren't for Lestrade," said John. "Most Wizards assume Muggles simply don't see anything worthwhile. They wouldn't have even blinked if Morton's investigation came to nothing."

Sherlock nodded. "And the truth was only kept hidden for so long because the investigators of the Magic and non-Magic worlds weren't aware of each other. When someone who had access to both worlds—Lestrade— took charge of the case, solving it became a matter of tedious legwork: he eventually discovered one of the victims had a wizard cousin, who will receive half a million pounds thanks to her death. That cousin, incidentally, is Smith."

http://www.fanfiction.net/s/7578572/42/