Police say they are questioning an individual over "suspicious envelopes". Two letters - one addressed to Barack Obama and the other addressed to Senator Roger Wicker - have been found to contain the poison ricin.
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The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.