Police in Brixton said the man they have arrested following a fatal stabbing where a 28-year-old woman was killed trying to protect her younger sister, is known to them, but would not give any more details.
Speaking this afternoon, Detective Reynolds said young children were at the scene of both attacks, which involved "extreme violence", and officers are trying to establish whether they saw anything significant.
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Police have raided another address in Moss Side as they investigate the Manchester Arena attack.
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Armed police were out in force as runners competed in the largest public event in the city since Monday's terror attack.