The FBI and the U.S. Postal Inspection Service are investigating suspicious packages that were sent to election officials in six states on Monday, the law enforcement agency said in a statement on Tuesday.

Election officials in more than a dozen states have received suspicious packages in recent days, triggering security fears fewer than seven weeks before millions of Americans go to the polls to elect the next president and Congress.

Threatening letters containing packages of non-toxic powders arrived at seven state offices Monday and triggered federal investigations.