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 US president enjoyed rousing support for his political agenda as a formerly hostile audience cheered his plans in office to the rafters.
The former leader said the party needed to learn lessons after his successor Paul Nuttall failed to unseat Labour in a hard-fought campaign.
Toddler Harry Studley was left blind in one eye after Jordan Walters, 25, shot him in the head with an air rifle last year.