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Dr Jagielska says the police are investigating the threats, and offered to arrange close protection, which she turned down due to the nature of her work.

The Met Police told BBC London the force was investigating a double shooting.

Authorities say Tyler Yates, another inmate serving a life sentence at the Sacramento lockup, is also being investigated for his alleged involvement in two suspected homicides this year.

A threat call which was made to the Glenanne Mill, where the victims of the shooting worked, days before the attack was not properly investigated by the murder investigation team.

The National Crime Agency is now investigating suspected criminal offences at the firm.