Greater Manchester Police assistant chief constable Steve Heywood urged caution over the 3D-printed parts found:
We need to be absolutely clear that at that this stage, we cannot categorically say we have recovered the component parts for a 3D gun.
What we have seized are items that need further forensic testing by national ballistics experts to establish whether they can be used in the construction of a genuine, viable firearm.
We will also be conducting a thorough analysis of computers we have recovered to establish any evidence of a blueprint on how to construct such a weapon.
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Scotland Yard said the male youths will appear in custody at Westminster Magistrates' Court on Saturday.
Windy this weekend with gales across northwestern parts of the UK. Rain spreading southeast.
Greater Manchester Police said "inquiries are ongoing to fully understand the circumstances of the incident."