Just before the rampage, which killed five people and injured 50, 52-year-old Masood declared that he was waging jihad in revenge against Western military action in Muslim countries in the Middle East after police managed crack into the message, The Independent has reported.
The recipient has been questioned and later freed after cops and MI5 concluded they were not part of the plot or had any prior knowledge.
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Kent-born Masood was shot dead during his murderous rampage.
The Westminster terrorist was previously known to security services and had once been investigated by MI5 over violent extremism.