The return address on the envelope sent to U.S President Donald Trump from Canada — containing a potentially deadly poison — included a postal code leading right to the Quebec apartment the RCMP searched last week, where traces of ricin were allegedly found. The surprising inclusion of such a potentially incriminating clue suggests the sender wasn’t intent on remaining anonymous for long, or masking their preparation for what a U.S. judge characterized as an assassination attempt. The strange letter inside, seen here for the first time, is handwritten in English in all capital letters. It is stained in blotches and...