Election officials in six US states have received threatening letters, prompting investigations by the FBI and the US Postal Inspection Service, according to NBC News. A senior law enforcement official told American media that no dangerous materials have been found in any of the packages received by election officials in at least six states. The FBI has urged US election commissions to remain vigilant following the receipt of threatening letters containing suspicious substances sent to several state secretaries’ offices across the country. The targets of these unknown senders include state sec…