Nikki Allan murder trial: Jurors shown brick from scene of Sunderland girl's killing
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The object was securely wrapped and was passed around to demonstrate its weight, which is over five pounds.
Summing up her conclusions, forensic expert Hilary Parkinson said she believed Nikki had been hit more than once with it.
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Hide AdShe said: "Nikki Allan had entered the Old Exchange building by the rear north west window.
"At this time she appears to have already been injured and been bleeding.
"The main assault on Nikki Allan has taken place in room three. At some stage when she was already injured and bleeding, it appears part of her body had been leaning against the east wall of that room.
"In my opinion, Nikki Allan had been hit with the brick and it appears she's been hit more than once with that brick.
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Hide Ad"Nikki Allan has been dragged from room three down to room one in the basement and she had been dragged for most, if not all, that distance by her legs or ankles."
Ms Parkinson added that forensics also looked for traces of sexual activity but found none.